Killstealing or ks'ing is not against the rules. Here are the in-game rules for FFH:
http://flyforhero.ucoz.com/forum/26-89-1
Gala-net made it able to ks/killsteal since v12. So it's not really a bug being abused, or anything else against the rules. It's just annoying, like Hnr'ers.
There nothing you can do against it, Killstealing/ks'ing is not against the rules allong with drop stealing.
Possible solutions:
-Try to lose the ks'er.
-Go to the PK channel and handle it there.
-Make a suggestion to add a rule that bans ks'ers.