How to unmount content?

Okay so get this first; I recently got Day of Defeat: Source and Left 4 Dead 2. DoD:S was done fairly quickly (no thanks to Steam) so I went ahead and started up GMod to use some content.

Stupid part: I went ahead and mounted L4D2 (which wasn’t even a quarter downloaded) and Zombie Panic: Source (old mod I uninstalled).
Now, Steam will load Garry’s Mod but it’ll never get to the main menu. Instead, Windows complains that hl2.exe is not responding. I have left this for about 20 minutes or so before giving up.
Okay I just need to get ZPS. No deal, right? (loads up ZPS enters a server plays a short game). GMod still doesn’t appear to be working. Well fuck, I have to wait for Left 4 Dead 2, then, I suppose. 2 hours, actually.

So, in a nutshell, does anyone know how to unmount content without entering the game, or if this isn’t the problem at all? I remember there being able to edit some stuff in a file some updates back to fix a problem with a GMod update, but I can’t remember what it was or what I should do. There is no crash dump either, GMod stops being responsive rather than stops working altogether.
A quick fix will help me better than the long, boring process of being unable to play any games on Steam.

yeah i’m an impatient fuck