How to tell my flatmate that we don’t want his girlfriend staying over every day?

Hey everyone, I need some advice on how to handle an issue with my flatmate. His girlfriend has practically moved into our flat—she's here every single day, cooking, washing her clothes, and basically living rent-free while the rest of us cover the costs of rent, groceries, and utilities. We’ve tried hinting that they should consider getting their own 1BHK, but they’re not taking it seriously. It’s becoming really frustrating for the rest of us because it feels unfair—we’re the ones paying for everything, while she enjoys all the benefits without contributing. How do we bring this up in a way that makes it clear we’re not okay with it anymore? We don’t mind occasional visits, but having her here daily is too much. Would appreciate any advice on how to approach this conversation!