If you stop him from seeing this girl, what about the next? What about when the next one is 19, 20, of legal age and giving him alcohol?

The bottom line is that YOU need to educate him about what he should and shouldn't be doing. About the dangers of alcohol, drugs, and unprotected sex.

Ruining some girl's life for what children that age see as completely harmless is NOT the way to go. You have to remember that though you see her as 18 and thus an adult, an 18 year old is no more mature than they were at 17. She is still immature and she probably doesn't realize that what she's doing is really that harmful.

To be honest, you need to not be scaring or threatening your son, but educating him. I still remember that age and the more your parents press things in the wrong way, the more you want to defy them. You need to talk the time to talk to him and make him understand.