Well my recomandation would be to make first ban 1 minute, than 2 days, than...
And maybe another suggestion, every lets say 2 months put everyones "ban status" one point backward.
So if one left first time 1 minute, next time 2 days, third time 4 days. But after 2 months lower his next ban. So if someone was banned for 4 days in 6 months he will again get the 1 minute ban. Still efective against mass leavers, but much better for less unban requests.