Here's what should happen.
Either (the easy way but not as good way)
Turn off Wither an sell nether stars to market now that they are impossible to get.
OR
I'm surprised you haven't done this, but just turn off MobGrief and get a plugin which disables a Wither's "eating" and initial explosion. Maybe there should be a trigger for moderators and above whenever a Wither is spawned so it is not used for griefing farms.
OR (the probably best way)
Make a big box of bedrock somewhere and make it kinda like the official Wither arena.