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 tightened, the round pin begins to raise.
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 and 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.