[AngleWyrm]

Give the users an option to have their oldest PMs drop off the bottom of the list. Then a new PM replaces the oldest PM, once they have 100.

It removes the need for maintenance labor from users (which is virtually guaranteed not to happen 100% of the time). And it could be set as a default for new accounts to minimize the problem.

Two options like so:
+ Replace oldest PM when inbox is full (100 PMs)
+ Refuse delivery when inbox is full (100 PMs)

Some users would prefer to maintain their own, though, so it's probably good to allow personal control of trashing for those who want to.