Joe, if they're not deleting the trash either the sysad or the ISP can. Or you can insure that they have a program (like Thunderbird) that will download EVERYTHING to their PC and they can sort it out. Then, if the trash gets extravagant it will slow down their PC and not the whole system.

That allows the sysad to yell at them and stuff, but it clears the mail off the servers.