I've seen a very good adaptive system in place for several MMOs in the past. Server load determined how long a character can be inactive before getting a 2 to 5 minute warning. If the character remained inactive after the warning, they get booted at the end of the warning period. Maximum load generally had a 10 to 15 minute inactive period, while low server loads allowed an hour or more. All times are given in ranges to cover the different MMO's I've seen this on.

Adding a check for 2 to 5 warnings in a row with less than 1 to 3 minutes of activity should result in a boot as well, for those who are blatantly trying to waste server space. I'm sure that implementing such a system would take some time, but I've got plenty else to do while I wait