The trick is to take a look at how the typical "molly" works. There is a long screw which goes through a non threaded hole in the front of the molly. This is the part of the molly you can still see when it is in the wall. The screw then goes between (usually 4) "legs" that attach to a threaded hole on the far end of the molly. When you stick one of these things in the wall, you turn the screw and it "sucks" the threaded end towards the wall, bending the "legs" in half and sucking them against the back of the drywall.
To undo this, drive the screw to almost all the way into the wall in the molly, and LIGHTLY tap the screw head to "unbend" the legs. back the screw out some more, and keep tapping until the molly gets loose enough to come out of the wall. The idea is to use the screw to pound the legs straight again, without punching the head of the molly throught the face of the wall.
In either case, you are going to end up with a fairly sizable hole in the wall. It is better to use several thin coats of repair mud rather than trying to do it all in one application where it is likely to sag or crack out.
My favorite wall repair compound is a product called "one-time" made by Red Devil. It is a water based product that is easy to apply and clean up is a snap!