So workshop content is saved in three places. Your Terraria mods folder, the steam workshop folder, and the steam cloud. Getting rid of it from all three places is sometimes just luck. However, the most important place to remove it from is the steam cloud, or else it'll keep re-downloading, or worse, will never let you download if it thinks it doesn't have to.

Make sure you're unsubbing from the mod via right-clicking tmodloader in your steam library, clicking properties, workshop, and doing things from there. This in theory should wipe it from steam, and the cloud itself. After that, you can try removing it from the mods folder which should be near the button "open mod sources" popup location.

Beyond that, all I can do is give you a copy of the mod to put in the override folder that'll ignore all other versions of OE as long as it stays were it should. Downside is you'll never get new updates.