EDIT: I misunderstood your situation hence my original answer. In the case that your shoe is still stuck to the cleat, remove the pedal from the bike first. Keeping the pedal on the bike will just restrict your motion as the crank arm and frame will get the in way.
Put the pedal in a vice grip or between two solid objects which allow you to keep the pedal still. Then simply twist the shoe in the normal unlocking direction until it disengages from the pedal. Having the pedal off the bike will allow you to turn the shoe a whole lot further and allow you to unclip. I don't suspect you would cause any damage this way.
Removal if it's just the cleat stuck (not on shoe)
Firstly make sure that the spring tension on the pedal is all the way down. This is lowered by turning the little hex / Allen screw to the left as much as it will go. Releasing this tension on the spring will make the job a whole lot easier.
Then use a set of needle-nose pliers to twist the cleat, simulating the twisting of your foot as you would when it's attached to a shoe.
If this doesn't work, you can try and use screwdrivers to pry the tensioned 'catch' away from the cleat, hopefully releasing enough pressure to twist it and remove it.
In terms of damaged caused, you may scratch the cleat and/or pedal, but I guess no more than you would by clipping in and out and walking around.