In a recent fight in my campaign, we were tasked with defeating a coven of hags. Not too difficult for our party of level 9 characters but we were held down for some time by their spellcasting. As is very common, many of the spells they were using required them to see their target. This lead me to a strangely simple counter to really any spell caster... simply putting a bag of any sort over the enemy's head and grappling them or using arcane lock or a similar non-magical lock to ensure the bag is not removed.
Would this work? If it does work and the creature has blind/devilsight would the bag obscure that vision?