If you are an MP: Change the law for the better. That's what you're for.
If you are not an MP, I would normally say write a letter to your MP, but I don't think that will do any good. Just be sure you vote at the next election for someone not already known to be on the take. Let's take their jobs from them.
You may want to join one of the facebook groups (search for "expenses"), or sign the petition to end expenses