Being reported for "Bad behaviour"?
Unfortunately, there is nothing you can do to stop people from reporting you for whatever reason they want.
Even if you get ganged up on and retaliate, this can still be construed as bad behavior.
This kind of thing is not limited to GTA. As Philipp Sander pointed out in the comment section, it's extremely prevalent in the League of Legends community as well.
Players will report you for any number of reasons, one of which is that they just felt like it at the time.