I have downloaded content from this site. But some of them I don't want to keep. Is there a way to extract custom content from the game and get rid of it?

You will need to identify the file in your Downloads folder (under the Sims 2 folder in your My Documents) and manually remove it.

Alternatively, you can delete Bodyshop items (clothing, hair, make-up etc) in Bodyshop. Objects you can delete in game (by selecting the object in question and clicking the trash can icon). Object recolors you can delete by first purchasing the object on the lot and then using the Design tool to delete the recolors you don't want. Walls and floors can also be deleted by just selecting them in the catalog and clicking the trash can icon. Hacks can only be removed manually outside the game.

You can also use the Clean Installer for this.