I read a thread on here a while ago that i didnt think would work, but it did:
Get an old credit card and cut it in strips long ways to the width of the holes on the head unit. Push them in slightly angling them outwards. When i did it it took a few attempts but you will feel them click and push past something (something being the sping clip holding the head unit in). Once you get to this point, slowly wiggle the head unit out.
May take you a few attempts but it saves doing the above and wrecking the headunit when taking it out! believe me i almost got to that point of just ripping it out, i know how you felt!