Iv made dozens and dozens of sets of slicks this way.... we used to run slicks at my home track when we ran our clay wet. Wet clay + slicks = better grip than carpet. The best way is to chuck a sandpaper wheel/drum into a dremel and dremel off all the pins. piece of cake.

Premade slicks and nowhere NEAR as good as a slick made from a pin tire out of the appropriate compound, with the pins ground off. You want the "texture" that is left behind on the slick. The pre-made slicks are too shiny smooth and don't work well.