The proper method is to gently pry upward with a flat head screwdriver as you unscrew them. They will lift out cleanly.
After you have unsuccessfully done that for 20 minutes, grab a pair of vice grips and pull the little effer's out. When you tear part of the air dam, look at it in disgust and satisfaction that you didn't want it anyway.
Remove the top frame cover piece quickly and easily using the same method. Then curse like a drunken sailer when you read the installation instructions for your new bumper and it says to replace that cover using the stock fasteners.
Then buy new ones from the dealer for about $8 each.