Sandpaper. Medium to coarse grit.
Pizza cutters are stainless steel, which doesn't sharpen very well to begin with and the fact that they're round makes sharpening even more difficult. They aren't Henckels
Maybe someone might sell an expensive carbon steel pizza cutter, but, for cheap stainless, a sharpening stone is overkill.
If you examine a brand new cutter closely, you'll see that the edge is rough and has been coarsely machined. Sandpaper will mimic this effect. Just make sure to do maintain the same angle and sharpen it as evenly as possible.
I wouldn't sandpaper them too many times, but I think a handful will extend their life considerably.