Kits

Not sure if I should have included this with my other post, but I'll just do this anyways. In my gamemode KitPVP, there is a selection of 6 different kits. The issue is, you can select all of these kits at once to combine all of their best assets into this one overpowered kit. How can I make it so you can only have one kit at a time, or if you select another the first kit deletes? Secondly, I need to figure out how to make it so you can only access certain kits within certain worlds.