Lowering the AC does not destroy Hyborem, but destroying Hyborem does lower the AC (is that what you meant to ask?)
Sanctify should work adjacent to any tile of hell terrain too, but it only effects the AC if it has city ruins within the range of it effects they are destroyed). I'm not sure, but I vaguely remember maybe having a problem using sanctify on tile that were actually on fire, but I think springing them then sanctifying them worked (and then springing again if you don't want deserts that will become burning sands again when hell returns)