I think a sensible thing is to give the TKed player an the option to forgive the tk. If the person doesn't actively choose to forgive, the killed skip the next spawn wave he/she is in queue for and get a + 1 in the TK counter. TKs within X seconds in spawn should be reflect damage and no other punishment.
What to do with the TK counter is imo more up for debate. Maybe with 3+ or so on TK counter, he /she is kicked from the server. If a person is kicked like this twice in a short amount of time, there is a short ban on NWI servers or something.