I've had the same problem recently with my Canon flash not budging when I tried to slip it off the hot-shoe of my Canon 40D body. I'm afraid I did use a lot of force to get it off...but I think it's still OK. And I think I've figured out the problem.
After reading the answers here, I took the ring-flash and...while staring at the bottom plate...began turning the tightening-ring back-and-forth between the extreme "tightened position" and the extreme "loosened position". In doing so, I could see the (forward-most, round) pin slowly go up and down. Seeing that, I now understand the (simple) mechanism that helps secure the flash to the body and keeps it from sliding off the hot-shoe.
When the circular tightening-ring at the bottom of the flash is fully loosened, the round pin is flush against the plate...and as the tightening-ring is tighened, the round pin begins to riase.
My mistake was not turning the tightening-ring to its fully (extreme) loosened position before attempting to slide the flash off the hot shoe! I was under the naive impression that the "cinching down" produced by the tightening rings is what held the flash in place. So, when removing the flash, if I turned the ring enough so that the flash looked & felt loosely-cinched against the hot-shoe, I figured I could slide it off. Not so!! Because the round pin only becomes flush with the base of my flash when its turned all the way to the completely loosened position.
Hope that helps.