I usually give out 3 warnings before TK.
I've only shot tanks that camp at base, who does no contribution to the battle, not even shout out orders.
Usually the fast moving tanks: M5 Stuart, M8A1, BT-7, T-28, Pz 3/4.. those who refuse to scout or help cover flank when allies are in need of help.
I would ask by typing in-game chat, specify location by ping, repeat the request.
By the time the request for help is refused or ignored and our team is up by 2-3 tanks, I would send out a final warning for TK, by then I usually have 3-4 enemy kills, TK fines do not matter to me.
On occasions, I would still get my TopGun award after TK.