Generally if I'm in the TA I check first to see if a trainer is in there and who they are.
If someone shoots at me without asking I politely ask them not to. If they continue to do so and there's not a trainer in there I just return the favor because it's generally someone who is very new and lousy enough for me to continually rain lead into. Then I giggle as they auger and ignore them as much as possible. If they come back for more lather, rinse, repeat, and giggle some more especially if they start losing it and cussing me on vox or 200. I might get upset if someone calls me something I haven't been called before but they're going to have to dig deep in the bag to find something and most of these guys have a big bag of nothing.
What makes this more fun is that, generally speaking, there are a few squad mates and some regs that will cheerfully join in the fun.
If there's a trainer in there I pm them and let them handle it.