Hmmm... I wish I were alone in having this issue. It turns out the solution to my problem was "cancel my ESO+ Subsciption and dissuade new players from buying Dungeon DLC." Doesn't seem like a particularly effective business model, but unless someone makes an add-on that implements a toggle, I'm better off this way.
Oh, and for anyone out there who may consider the XP buff to be a special reward for "serious" players, please note that I now enjoy the XP buff for doing just random normal dungeons, exactly as it would be if I had flipped a toggle to turn the DLC Dungeons off -- and I'm saving real-world money while doing it.
Gwynauldynn. Crafter. Merchant. Guildmaster. Adventurer. Uthastyan. Corporal of the Lion Guard, abroad for the glory of the Covenant. Dra'arah. This one picks pockets. Sleeps-With-The-Fishies. You want a soul gem filled? Pheh. My pleasure. Vanyonah Telvanni. Born an ashlander. Destined for greatness.