Do as the following steps:
1. Wash and dry the lenses.
2. Dip a cotton swab in denatured alcohol. Allow any excess alcohol to drip off.
3. Apply the tip of the cotton swab to the logo on the lens, being careful to touch only the logo on the lens. Swirl the end of the swab in little circles. Repeat if necessary with new swabs until the logo is removed. Wash the lens with soapy water, rinse and dry.
Some designer sunglasses with printed logo on them are samples for customer to choose. If it is not a sample, you can try some rubbing alcohol to remove it, but it may leave some marks on it. It is better for you to take it to optical store and ask them for help.
I have saw such question somewhere i can't remember. But i still remember that someone answered the question -Tell your girlfriend to do it with some of her nail-polish remover. Should come right off . Maybe you can have a try.
But i saw many brand sunglasses put their logo on sunglasses frames not lenses. why not choose such brand sunglasses with logos on frames ?
MR. CLEAN MAGIC ERASER. I just spent the last hour trying everything--rubbing alcohol, acetone nail polish remover, peroxide, whiskey (which, scarily, was somewhat effective). I even called around to some vision places and they all told me just to pop out the lenses. But then I wet a paper towel in warm soapy water and let that sit on the lenses for 10 minutes and rubbed with a Magic Eraser and it came off! You have to rub hard and fast, but it was gone within seconds.