If you don't want to use certain DLC that you have purchased like DLC dungeons can you turn it off? Is there a way to do this? I have not found it yet.
No way to turn them off.
You've buy them, you don't want them, so don't play in these areas. And if you're using ESO+, just wait the end of the subscription, then they will be locked again.
It's like shoes, you buy the product, but finally you don't like them => No way to take it back to the store, so you just leave it at home without using them ^^
Don't worry it's not affected your game. Even when you don't buy DLCs or using ESO+ you have these zones ingame and in your database, it's just that they are locked.