Put a few more layers of nail polish over it... but be sure to wait until one layer dries before putting on another layer. Then, aftera bout six layers of polish are dry, or it's just really thick, you should be able to peel it off.
If you don't want to do this, I've found that if you just put one layer of nail polish over your current nail polish, and wipe it down hard before it dries, both layers will come off. I don't know if this works every time though.
Hope I could help!