On the other hand, I'm not sure I really want to discourage people to help in this game-- I'd much rather have them help me fight than really be in trouble and have them /laugh as I die. So I'm not sure I'd tell everyone to lay off.
Rather, the problem here is that when I pull a group of mobs, I don't want someone else stepping in and tagging them away from me in the name of help. So there are plenty of ways to help that don't include stealing my XP-- drop a DoT on something that's been tagged (I do this all the time on my Shadow Priest), or throw me a heal (always appreciated, even if my Mana Tide totem is off cooldown). And just paying attention is nice, too-- nothing burns me more as a Rogue than taking the time to sneak up on a mob, only to have them charged by an overzealous warrior who claims afterwards that they didn't see my Distract go off.
If it's legitimate kill-stealing, the best option is just to ignore and move on to a less populated area-- there's no dealing with those people. But if you do see someone fighting a bunch of mobs solo, there are better ways to help than grabbing one of their kills, and chances are they'll be more than happy to help you the next time around.