Reset a player's progress on a server

If you remove the files in the world\players and world\stats folder, the server will forget that they were there. If this is a bukkit server, there is a players folder in each world. Files are named with the ID of the player, so you can delete selectively. To get the ID of a player, open https://api.mojang.com/users/profiles/minecraft/(playername) (substitute (playername) with the actual player name)