When you have a reactive dog (particularly, though not exclusively, with a dog who may become physical [bite/scuffle] to get the scarey thing to go away or to express frustration/arousal) it is of the utmost importance to advocate for your dog.  Dogs that live in a society governed by leash laws do not have the opportunity to flee or add distance, so they are subjected to the grabby hands (or the “he just wants to say hi” type dogs)  of the world whether they want to or not–that is, unless the human handlers steps up and advocates for the dog.  It is really inconceivable how some handlers …uh, handle, situations with their reactive dog–or rather how they don’t.
Just say no!
A few weeks ago I saw an interaction between dogs/handlers that blew my mind.  Dog A is a young dog who is dog selective/has space issues/is reactive/is a control freak–I don’t know which, but I have seen multiple reactive displays along with some lunging and growling at dogs who enter her space.  Dog B is a toy-breed dog who is extremely friendly (“he just wants to say hi”)… he is a dog who Shayne struggles around… very bouncy, very energetic, very spirited movements–an absolute life of the party type dog.
Dog A was casually standing around standing around.  Dog B was walking by and started bouncing/pulling/whining/dancing (on back feet waiving his two front feet) toward the other dog.  Dog B was about 6ft from Dog A.  Handler B smiled at Handler A, who reciprocated the gesture.  Handler B finally asked, “Is your dog friendly?”  Handler A replied, “Oh, yeah, she’s friendly.”  Now, there’s me watching this go down… I was shocked that Handler A said that the dog was friendly (with the implication of letting the dogs greet).  I know Dog B and my instincts (from watching each dog in previous situations) told me this wasn’t going to end well.  Sure enough, after a few seconds of a tense greeting, Dog A took off after Dog B.  Both handlers pulled back their dogs, Handler A scolded Dog A and Handler B swept down and picked up Dog B.  Now there was no intention to hurt Dog B, but it was certainly not friendly or appropriate (I was quite bit away handling both dogs, focusing on keeping Shayne sub-threshold since I expected a commotion).
I hadn’t seen Dog A too much, perhaps 6-10 times, but I had seen this dog have similar reactions/snarkiness/over the top correction to dogs probably 5-8 times (never to this extent, typically a growl and snap but not the physical attempts to get on top of the other dog).  In my opinion, there is no way that Handler A was unaware that Dog A was indeed not friendly (in this environment and under these leashed circumstances).
To me it was completely unacceptable for Handler A to put Dog A in a position where the dog felt it had to get snarky/defensive (a situation that was not out of the blue).  Handler A not only put Dog A at risk, but also Dog B and Handler B.  Poor Dog B was screaming and very scared.  Now Dog B was overly friendly and was not being the most appropriate, but Handler B asked if it was okay before allowing the dogs to greet.  Handler A had all the opportunity in the world to advocate for Dog A and protect Dog B but chose not to–a simple “no” would have prevented the entire situation (since Handler B is someone who takes ‘no’ seriously).
It is really so very important to advocate for the dog and learn to say no to people who ask to pet or allow dogs to greet.  Preventing reactions is just as important as keeping the safety of other dogs/people in mind (even if it’s just mental trauma and not physical injuries).  Although Rio is friendly during leashed greetings, I very rarely allow him to greet another dog on leash.  Pretty much the only time that happens is when it’s a dog of a handler I know well and trust their judgement.  I use many different techniques to communicate that I want no interest in letting your “friendly” dog greet with Rio (and certainly not with Shayne).
In my classes I actually have students practice saying, “Sure you can pet my dog but he/she has to be sitting first” or “No, he/she is a little shy,” or “Yes you can pet but first put your hand down like this and let him/her touch your hand,” or “Well, he/she doesn’t like to be pet but you can toss these treats on the ground (alternatively, “you can watch him/her do a few tricks”)” or “Sorry but he/she is not friendly!” (however untrue this statement may be people normally listen to this even if they don’t listen to the shy/scared comment).
It’s my job as any dog handler, but particularly that of a dog who has some issues with new dogs, to keep my dogs safe while considering the safety of others.  Part of what that entails is communicating with people what is okay and what is not okay in regards to my dog–this keeps her safe and this keeps the other dog safe.  Some of the ways I communicate to people that my dogs do not want to be pet (or greeted by their dog):
–If the person is at a distance and just starting to approach, I try to avoid major eye contact and then walk away from the person/handler approaching casually–simply increase distance between me and the stalker sometimes is all I need to do to end the pursuit
–If the person is a little closer, I’ve grabbed my phone out of my pocket like it just rang and, after normal pleasantries, started having a heated argument to an imaginary person on the phone! (sometimes I pretend to have a non-argument conversation but heated arguments tend to make the other person leave… sometimes real stalker-types will wait through a normal phone conversation)
–I frequently say things like “She’s not friendly, but thanks for asking” or “she’s really shy” or “she’s scared of strange dogs”  in a pleasant tone which often works
–I’ve had to resort to saying “Stop!! Keep your dog back!  She is not friendly!”  or “No, really, she doesn’t want to say hi to your dog, she does NOT like other dogs.” However untrue these statements may be, if it prevents an idiot dog handler from continuing to allow their dog to approach, it’s worth saying.
–I’ve planned escape routes from walks so if we encounter someone who potentially doesn’t listen (or who has a history of not listening) I can do some evasive maneuvers to get out of dodge.
It may sound silly but I think it’s important to practice saying these things or doing these things so you are prepared if you ever need it because it is important to advocate for our dogs.