Don't plan to re-use them. See #4 in the right hand column here: http://chevy.oldcarmanualproject.com/gmc/5559mm/5559mm005.html
Place a brass punch dead center and hammer away. The edges will fold upward and you will be able to pluck them off with a flat tip screw driver. Just did it a week ago. Went very smoothly. Also, you can drill a small hole in them and use an any lever type rod that will fit in the hole to pry it up. If you plan on re-using them, the brass punch will work best. Just hammer them smooth; but it's best to replace them