Cecil Adams gave a response to this problem (as if he'd every leave a question unanswered if he could)
A touchy problem, to be sure. I recommend a remedy prescribed by the 14th-century physician Bernard of Gordon in his Lilium medicinae:
Finally . . . when we have no other counsel, let us employ the counsel of old women, who may slander and defame the girl as much as they can, for they are more sagacious in this than men. . . . Let there be sought a most horrible-looking old woman with great teeth, a beard, and evil and vile clothing who carries a menstrous napkin in her lap. And, approaching the lover, let her begin to pull up her dress, explaining that she is bony and drunken, that she urinates in bed, that she is epileptic and shameless, that there are great stinking excrescences on her body, and other enormities concerning which old women are well instructed. If the lover will not relent on account of this persuasion, let her suddenly take out the menstrous rag before his face and bear it aloft saying with a loud cry, "Such is your love, such!" If he doesn't relent on account of these things, he is a devil incarnate. His fatuousness will be with him finally in perdition.
Let me know how it works.
A LATE SUGGESTION
A canny female friend suggests another approach. Persuade the useless girl to say to her besotted swain, "I love you--I want to have your baby."
He'll be out of there like a shot.