How to get through to my EX wife?

My EX wife Stacy is dating a guy who i can't stand. He treats her like crap and according to my 5 year old daughter he is mean to her and hurts mommy. Stacy told me he seems like an asshole but he isn't. She told me it's none of my business who she dates and she doesn't interfere with my personal life. It's been a 3 years since the divorce and i have a slew of meaningless relationships nothing serious.I still care about her i always will she is important vital to not just my life but to our daughter as well. What should i do?