If you are going to have this 48 hour rule, then the UserCP should list the previous 48 hours of Reputation.
This is just common sense and fairness for everyone.
Common Sense would preferably dictate disabling the Neg Rep ability, for 48 hours so you couldn't neg rep the Poster twice within the time period. It would beat punitive and permanent damage zeroing out your rep and knocking you out of the rep system completely.
My suggestion was to have your Neg Rep go against the User, rather than the Target, automatically, if you neg repped the same person too often within the time period. I thought it would be fun to watch the reactions as the realization set in, that abusers were knocking out their own rep.