If you put the 20 year old on notice to leave your daughter alone, and he persists, then you might be able to pursue charges for contributing to the delinquency of a minor, otherwise the protective order is the way to go.
Slim chance you will get anyone to admit to any sexual relationship that would amount to a rape charge.
If you haven't done so, call your local police and inform them of the matter. You might be able to persuade them to go talk to the guy... and that just might be enough to spook him away for good.