My players have broken me...

So...one of my players asked "is there a way to turn the water in [strahds] blood into holy water and just....microwave him?" This then turned into all 4 of them brainstorming how to turn lake zarovitch into holy water, and make the air around toxic to all undead. I don't want to rain on thier parade (lol pun intended) but I also don't want to let these chaotic lvl 5 warlocks just run rough shod over my campaign. HELP