I agree with Dk, only to the degree that if: its a player who lags, and stubbornly keeps connecting despite warnings from admins and kicks by admins(not the server mind you) that he/she should be banned for 30 minutes. So if a player is laggy, and he gets kicked like 5 or more times by the admin, and they tell him each time to fix the lag, but said player keeps connecting everytime after being kicked without fixing his lag, yeah i'd ban the guy for like 30 minutes cuz of lag, but nothing more than that.
Other than the above scenario^ i don't think anyone should be banned for lag, kicked from the server sure but only by the admins, and or if they have ridiculously high ping.(higher than 500 i believe is what Matt stated)