I agree with albaqwerty, but oil based and water based paints will behave somewhat differently. What possibly would be of help would be to apply something like white spirit first and allow it a chance to soften the paint for a while (repeated application may be justified). Whatever scraper you use, be certain it is not too sharp because using something that is quite tricky to use if the plastic is not to be scratched/damaged. Try on the least conspicuous are first to see if any discolouration takes place (shouldn't) and to perfect the technique.