I use a plugin(which i can share) that , you set the tk limit, i have it set at 4.
If someone tk's, before it "counts" as a tk, the victim can choose. If the victim picks ANYTHING other then forgive, then it counts instantly as a tk. If they do forgive them, it obviously isn't counted.
What happens is, say you want to blind your attacker, but you are the 4th person he has tk'ed and has not been forgiven by, the second you press "blind" it kicks him, because i'ts counting as a tk.
I have my limit set to 4, and it bans them for 30 minutes. That way, if someone rpg's into 2 or 3 people, and say's sorry, they can be forgiven, if someone just goes on a rampage and tk's 6 people after the spawnprotection wears off, they will most likely be kicked/banned in about 3 seconds(as long as it takes the people to punish/not forgive them).
This system is more flexiable then just counting everytk as a negative one, as people can say sorry and explain before it counts, or have it removed from the tk count as is.
Also, there is a system with say, 2 or 3 kills, you get 1 tk removed.
The minus points for a tk only punishes those that care, and are not doing it on purpose, it could be -1000 and people who want to tk still would...it's a real shame.