Well, if you are trying to prevent/discourage spawn-farming...
How about this:
Player A kills Player B
-> Upon respawning, it takes 60 (or more?) seconds for Player B to return to full kill-value (5xp added to his kill-value every 3 seconds)
with relation to Player A
-> However, it takes only 20 seconds for Player B to return to full kill-value (5xp added to his kill-value every second)
with relation to Player C
What would this do?
It would discourage people that are already camping/farming, because campers would see a
sharp drop in their ratio of:
Point gain
Time spent camping
However, it would also discourage players like Player C from joining in on the farming, because even if he was to participate briefly, the point gain would be meager.
But of course player C shouldn't be punished for not partaking in the farming process, which is why the timer for him should be shorter. It could even be as short as 10 seconds, but that is a balance issue.
What this would do is discourage the zerg by creating a sort of invisible "bubble-of-low-exp" around the spawn area. This means that spawn camping would mostly occur for tactical reasons, and if tactics are already involved, the organized group might choose to just take out the spawn-generator instead.