I've done it before just by hitting them with a claw hammer breaking them off bit by bit. Once you get a little bit off you can get a flat screwdriver and "chisel" them off gently, getting the screwdriver behind the tile and tapping with the hammer.
When using the screwdriver, like this, try to keep it at a shallow angle to the wall.
Wear goggles and gloves if you try this method because they will produce very sharp shards of tile.