Kill stealers are there, and they come in two flavors. You have the new players who are just clueless as to how things work, and mearly see a way to have fun. The real problem are those who know all too well what they are doing. These are the bozos who chase the bomber with a wing missing, floating down to the ground in a death spin, and do everything they can to get the kill for themselves.
These players are not playing in the spirit of the game, and do not promot a positive and fun environment. There are a few people who I will not aid in any way because I have watched them over and over trying to steal kills. I tend to politly mention to a player the first time I see them blatently attempting the steal (shooting tailess planes is a dead givaway), after all, you never know your doing something inapropriate if its never mentioned to you in a polite and respectful tone.
One important caviat to this discussion is that I do not consider someone shooting a plane I am engaged with as someone trying to steal the kill. It is certainly not always my favorite thing to have "help" shooting down a low, slow, and inevitably doomed adversary whos six I am glued too, but I certainly would rather have help as a matter of regular habbit, than die because someone was worried about being accused of some transgression. There is a huge diffrence between helping kill an active threat, and pumping lead into a doomed piece of falling wood and metal.
One thing I tend to do (and would hope others adopt as well) is to hang out above a fight that I see my teamate has well in hand, and make sure he doesnt get jumped and also be close by just in case the tables get turned. Not only does this help protect them, but it can also distract their target and make the kill easier for them. The long term goodwill, and cooperation gained from assisting your teamates will provide far more satisfaction and a much higher scores than taking that one kill does.
If your playing Aces High as a solo player, and not a team game....your not playing Aces High and your missing the fun!