To un-install anything from the Sims, go to the tab 'Installed Content' in the game launcher. Next to each item there is a box. Click the box of each item you want to delete, then click 'Uninstall' at the bottom.
Custom Content (hereafter referred to as CC), however, has a tricky definition, especially 3rd party CC. Technically, anything not found within the base game or any expansion pack/stuff pack is CC. That includes Store content and items found on the Exchange.
Some CC is created by EA. (Store Content) This type of CC is usually safe, since it is created by EA. The forum does not have a problem with Store Content.
Some CC, however, is made by somebody other than EA. (3rd party CC) This type of CC can often be harmful to your game, since EA does not create it. When we say that we don't want CC here, it's 3rd party CC we're talking about.
The Exchange contains Sims, patterns, houses, worlds, et cetera, created by players. Sometimes these creations contain 3rd party CC, which can harm your game. I would suggest not downloading anything from the Exchange unless you are certain that 3rd party CC is not present in the download.
So, techinically, items on the Exchange are CC. But only items on the Exchange containing 3rd party CC is generally harmful.