That's a great question.
I'm thinking that you may not be able to fix them. Since these tend to be either a rubber or a hard plastic, you could try taking some fine sand paper to them. But that's going to change the over all texture. Or...and this is a big if and I don't think I'd try this on a customer's car but maybe my own...take a heat gun to it very carefully! As you don't want to heat up your paint.
But again I wouldn't personally want to do this to a customer's car. I'm sure someone will with more experience with this type of thing will chime in and point you in the right direction.