My daughter is constantly flirting with her step father despite me talking to her about how it makes me feel. How should I react?

When my children were little I lost them to the CPS. After 16 years my kids have found me. Which for me it seems my prayers have been answered except the fact that my daughter now 18 has a crush and is constantly flirting with her stepdad. I have talked to her a lot about it and told her it needs to stop. Anyone else I would have already rocked their head. But she is my daughter but doesn't seem to respect me at all. What should I do?