You are a devout, perhaps even saintly follower of some religion. Your spiritual purity is such that it drives away the powers of evil to a distance equal to your Magic. Setting up the ward is a 3-shot action, and maintaining it is a continous action.
Any SupernaturalCreature attempting to break down the purification must succeed at Magic checks, with your Magic as the Difficulty. If they fail, they are not only unable to enter the area or affect anyone within, but suffer WoundPoints equal to the negative Outcome of the check. If the creature succeeds, you have the option to either abandon the ward (in which case it goes down) or spiritually maintaining it (I which case you take WoundPoints equal to the Outcome).
If you have a ward going, you can force a creature back using the power of your faith, in which case the creature must immediately make an Magic roll as outlined above. In this case, it can elect to actively resist (a 1-shot action) to get the usual +3 active defense bonus.
Even if your ward is broken, you can set up a new one later.