Personally I wouldn't really worry about this, I mean it's a glitch because something wasn't setup right, if they aren't able to get hurt they aren't able to use it for advantages.
I don't really find this illegal, it's more like reporting a glitch, but the users of it should of reported it before, first check and see if they are protected or not, because if they are in a non pvp zone their arrows will be deflected and not effect others in pvp. If it's a pvp zone then changes the regions up; just let an admin know.
Get a admin or tech to disable or check out that regions protections and commands; maybe add some new blocks.