How do you make TNTWars include placeable TNT but prevent the TNT from marring the landscape?

Set up an arena, add some extra blocks to it and create a custom dynamite block. You'd need a custom plugin that makes it turn into regular dynamite after a few seconds or specific actions, which would then cause an explosion (with blocks that are entered into the system and cannot be broken). I think you should ask icanttellyou, he might be able to create that plugin for you