You can't really leech in cRPG, because you will gain gold and xp no matter if you are dead or alive.
I prefer to call it grinding. You can wear zero upkeep pesant gear, suicide attack every round from lvl 1 to lvl 31. That is hardcore grinding. Not leeching. Unless it is being done with the chat command, which would make it abusive grinding, which should be kickable if done repeatedly.
Most important is that players don't delay the round. Not to prevent people from appearing afk by forcing people to instantly move. Admins can ofc give a warning (and should) before they eventually kicks but you should never be kicked without a warning just because you appear to be afk. As several players have already said, it can be a good tactical choice to "wait" or if you are hiding, to stand still.
And sometimes you actually need to go afk for 20s...
So I think the 30s no movement rule is enough (but you should get auto kicked, not reported). You should however always be kicked as last man alive if you are standing still for more than...5s, without a warning.