how can you remove ascratch on plastic eyeglass lens?

If you want do it yourself, use toothpaste. Put some on a soft rag or chamois and rub lightly. However, you don't say if your lens have anti-reflective coatings. If so, I advise you take them to your local optician who can replace the coating if required. Alternatively, take them back to where you bought them, or find a dealer near you from their website - http://www.crizalusa.com/Pages/default.aspx but any good specs shop should be able to do it for you.