How to remove addons from multiplayer game that somehow got into singleplayer?

If you're confused about the title, basically I was playing GMod with my friend and he got a lot of addons. I do not know how this possibly happened but somehow all of those addons were in my singleplayer game. For clarification, I did not install the addons manually and instead used Ultimate Workshop Downloader. How would I be able to find and delete these addons?