My step-daughter’s (19F) boyfriend (20M) hit her, so I (37M) kicked his ass

My step-daughter (Lilly) went out with her boyfriend (Mike) for valentine's day. Later in the evening, I was watching a movie with my wife, when Lilly came in. She was crying, saying that Mike was drunk and wouldn't get out of the car, so she tried to unbuckle his seatbelt and he said "don't fucking touch me bitch" and slapped her. I went outside to see what was going on.

Mike was in the passenger seat of the car. He was laying back in the seat, was awake, but his eyes were closed. I told him to get out of the car, come inside, and let's talk. He wouldn't get out, so I unbuckled him; he started trying to punch me, so I grabbed him by the shirt and forcefully removed him from the car.

I stood him upright and told him to calm the fuck down. He looked at me for a second, and then threw a wild haymaker at me. I slipped under his punch and grabbed his legs, picked him up, and slammed him on the ground. He landed on his head, and he was out cold.

I dragged him into the house, where he shit his pants about 10 minutes later. When he woke up, he said he didn't remember anything from the night before, and Lilly believes him.

Lilly wants to stay with him, but I do not want him around her anymore. I am afraid that if I expressly forbid the relationship, she will "run away" with this guy. I feel like I'm playing a zero-sum game. How can I get her to leave this guy?