Okej so lets say you want an arena where they can build, pvp but they cannot bend in there. So what you wanna do is

you make a region:

than you wanna flag this so they can build in it: /region flag [region] build allow

but since this will allow the bending you will need this plugin: http://dev.bukkit.org/bukkit-plugins/rsp-region-specific-permissions/

so when you have this plugin you can go on with deny bending in this region, I kinda forgot how exact you do this but I think you got a new "flag" like the deny-command flag you now also get deny-permissions flag so you will add to this region that flag and than which permissions aren't allowed there and to disable bending fully you'll add -bending.*

* Oh and the plugin won't use the pvp flag since it gives a lot of problems. I hope I helped you with this else just PM me and I'll try to help you further.