Is the sail Dacron polyester? Are the letters colored Dacron with sticky back adhesive? If yes, the letters are made of deacon insignia cloth.
You can just peel them off. You can clean any residual adhesive from the sail with acetone and clean *white* cloth rags. Use a clean rag for each swipe, so you don’t smear the old adhesive around.
The acetone will not hurt Dacron sailcloth. but don’t get acetone on any plastics. It can melt some types of plastic.
Judy B
Sailmaker.