A pizza box, or any paper product, that is stained with grease or food can't be recycled unless you remove all of the contaminated portions. The only way how to recycle pizza box is to tear or cut the dirty areas of the box and trash them. If the whole box is clean and grease-free, you can put it in paper recycling. But make sure it is clean. Otherwise, it can contaminate an entire load of recycling. If greasy areas can't be separated, the box belongs to a household waste bin.