I personally think that warnings should fade over time. For example : after not having received another warning within 2 months, 1 warning fades etc.
I also think that people that have received warnings should be given information on why they where given a warning. This could be done with text that appears on the "My Account" page, or with a sort of message of the day system that appears everytime you log in to the game. If you don't know that you've gotten a warning or don't know the reason for it, its hard to adjust.
+ Pikachu made my day