If you don't have anything good to say, then they shouldn't be able to say it.
Remove the negative aspect. Promote the positive.
WOWS automated systems can apparently detect AFK and disconnected players, so the AFK option isn't relevant.
WOWS team kill penalties have recently been improved, so "unsporting behaviour" is less relevant.
Misbehaviour in chat is too subjective, and shouldn't be an instant karma strike, but rather be reviewed after x number of complaints, and (e.g.) account suspension applied if found offensive or rules broken. (But again "offensive" is subjective). Personally if they rant/swear I don't care, but then I'm old enough to not let that affect me. Maybe younger players aren't?
If they really are that bad a player, then their stats will show it, and if a more balanced random team lineup algorithm is used then each team get a noobish dd player (e.g.).
I'm getting negative karma even after coming (near) top of the team, supporting the team, not spamming the chat, or doing anything I can think that would offend or deserve a negative karma strike. While I don't really care, it just stands out as broken and should either be removed entirely, or fixed.