Undestroyable Map

So I have this PvP Arena, and I want to add, for example, an ability that makes a barrier by placing a mini wall of blocks in a certain place. The problem is that, once the mini wall dissappears, it may have destroyed part of the arena due to then/fill command. How can I make the arena, or simply, how can I make a block stay persistent, not being able to be destroyed or swapped, without using a ton of /fill commands constantly ticking?