Install the L4D2 Authoring Tools, then open it to see this menu:

Open the Hammer World Editor, then either create your own basic map or load one of the various templates it comes with.

After performing Step 2, run the map through the editor, and let the game load into it.

Now, close the game, close the Hammer World Editor, and close the Authoring Tools. Once you've done that, open L4D2 like normal. You'll now find that all of your workshop addons have been deleted and must be redownloaded!

Honestly, who ♥♥♥♥♥♥' knows? I know I don't. It's the Source engine man, this ♥♥♥♥'s weird.

I made this only because I did this by accident and learned the hard way, and I found it to be hilarious. I highly recommend you DON'T actually follow this guide if you don't want to waste an hour redownloading and making sure everything you don't need is disabled.