I have 8 years old kid and I don't know how I would react when he brings his girlfriend (when he is old enough) at home and I don't like her. However, I might try to understand my kid's girlfriend and analyze whether she is good for him or not.
I focus on understanding why I feel that way and keep communication respectful. I share concerns calmly with my kid but avoid being controlling. I’ve seen openness help improve relationships or at least keep peace. Ultimately, it’s their choice.
I'd simply talk with the two of them and get to find out why he/she acts inappropriately then if he or she is teachable, then I'm willing to teach and correct patiently knowing that no one is perfect. If he or she is not teachable, I'd expose my kid to the danger ahead and move on with my peaceful life. My child has to decide if to continue or not. He's the one that will face the consequences not me, I only need to cope.