What it says on the tin. My dragon champion boss monster pretty much burned himself to pieces without the player character needing to do anything, since burn deals damage by % of HP per tick. That was the problem with Dredmor before, I believe - it was too easy to take him down with burn. We already have cannot charm and cannot paralyse flags, so why not cannot burn? That way we can put burn back to 5% of total hp per tick, so it doesn't feel so useless on things that are supposed to be that way.