A fine Scotchbrite pad can often come in handy but they're far more aggressive than you might think so go with the finest you can find.
Before using any sort of physical force try diffferent solvents, MEK cuts most anything, acetone, toluene and xylene are also good. Kind of depends on what type of plastic you're trying to dissolve.
If none of those will cut it go to a plastics place and get some Weld-On #3 or #4 solvent based glue. It's not only extremely aggressive but it's very handy to weld plastic pieces back together with. It really isn't a glue but a special solvent that is used in the manufacture of many plastics. With some care, a hypodermic needle setup and/or some toothpicks you can mend a lot of things nothing else will even touch.
You can order it off the internet but it's a heck of a lot cheaper to buy locally, most any place that sells Plexiglass or Lexan in bulk will have it.