Geodata Or Java?

Hi boys i have some problem, i want make coliseum like admin pvp zone. that means inside on coliseum can be only persons which admins teleport.

Problem is that i need to make some walls around coliseum that normal players can go inside.  Simple example like Olympiad Arena, can't go outside of lines. How i can do this because i try play with geo editor but maybe i dont know how to do it. 

How it must to look like i put on image bellow, that all player around arena and they can not get inside.

Thank a lot.