Kill stealing happens but here are a couple pointers.
1. Call your kills! I really wish more people would get into this. It prevents other people from wasting their time and energy chasing a plane that is mortally wounded and helps prevent kill steals (plus it's good for the ego). Mortally wounded would be defined as;
a. No horizontal or vertical stabilizer
b. Missing one entire wing.
Even a deadstick or clipped wing plane can still be a threat and should not be called dead. Although I request and encourage people to call "I got him under control" or similar while finishing them off, again, to prevent other people wasting time and effort and keep them from shooting "your" kill.
2. A polite, but public, reprimand on range vox usually elicits an apology and helps train people to not kill steal (at least from me).
I dis-like kill stealers as much as anyone, but I always keep my cool by remembering that most of the kill-stealers are skill-less noobs and this is probably the only way they can bag kills.
Pity the poor kill-stealin', skill-less noob, for they know not what they do