Person A uncovers & reports 1200 examples of Group B breaching professional guidelines. Supporters of Group B say A has a vendetta without addressing the fact that breaches are occurring. What fallacy are the supporters committing.

Person A has a particular interest in professional guidelines and has investigated the advertising practices of members of Group B and as a result made 1200 complaints to the professional board for breaches of the advertising guidelines - namely making unsubstantiated claims. Person A is not a member of Group B. Supporters of Group B are saying the A has a vendetta against Group B and therefore the complaints are vexatious. It is worth noting that thus far all of the complaints have been upheld. I say that B’s argument is fallacious: while he may have a vendetta, the complaints are legitimate. I am not sure which fallacy is being committed. I am thinking Red Herring. Your thoughts?

I'm not sure but I'll guess. It seems to me that claiming A has a vendetta is a type of attack on his character. If this is true then it would be an ad hominem if they are saying the complaints are not legit because of the vendetta or behavior of A Of course I'm just a hack, so better wait til Dr. Bo chimes in.

I believe this is called an Ad Hominem and also a Red Herring. They are trying to attack you (ad hominem) by saying you have a "vendetta" and distract the reader (red herring) by diverting attention from your facts and onto a completely irrelevant topic(s).

This happens when the other side has no valid counter argument or proof to disprove your claims. It basically means they're f@cked and they know it, so they're trying to divert and/or downplay everything.

Keep poking where it hurts them, and turn the screws so tight they scream in pain and give in.

This sounds like a version of the ad hominem fallacy (or abusing the man), because it attacks the motives of person A without addressing the argument. I am assuming that person A's argument would be something like this:

Premise 1: A Group that breaches professional guidelines should be sanctioned (or punished or whatever).
Premise 2: Group B has beached professional guidelines 1200 times.
Conclusion: Therefore, Group B should be sanctioned.

This is a valid argument. The only appropriate response would be for Group B to questions its soundness, i.e., whether there really is evidence of 1200 breaches of guidelines or whether that number is really sufficient evidence of "breaching." Maybe that's a low number for that industry.

But, to simply say group A has a vendetta, is not an appropriate response.