reward/penalty problem.

i want to make a kinda new hack, combination what i've seen around here.

Mods/admins will have the ability to give a certain user a penalty/reward if they act on a certain way in the forum (spam/helpfull).
That part isn't hard (new table, new script, alter postbit in current script).

Yet, after a certain time the penalty/reward has to be auto-deleted (lets say after 1 week). That without interferance of a mod or admin.

What is the best way to achieve this auto-deletion?

I'm starting this script tomorow, hoping i will finish it in one day, so i can convert it to a good addition for the hacks