In my room mate days, one of them had a gf that started to spend a little too much time there. Here are some of the situations I came across:
1. When she would come over, they would cook together and then not clean the kitchen. My room mate doesnt cook, so he hardly uses the kitchen. But when she was over, the kitchen would get trashed and then not get cleaned. They would go nap or go out after using the kitchen. I asked him to keep the kitchen area clean but my words went on deaf ears. So when I would come home from work and see the kitchen dirty, it would mean that they had just cooked. I needed to cook myself dinner, but I shouldn't have to clean their mess first. So I just went into the fridge and helped myself to their food. Even if it was the last of what they had cooked, I would still finish it. I would clean my dishes and put them in the dishwasher and leave their mess alone. Later when my room mate would go looking for the food they made, I'd tell him I ate it and explain that the kitchen was unusable until it is cleaned. And since I won't cook in a filthy kitchen, any food is fair game.
2. I also stayed in my room alot when they occupied the living room and eventually I started too resent this. I shouldn't have to feel trapped or uncomfortable in my own place. So I started walking around in my boxers. I would cook, clean, eat in just boxers. She would see me and get uncomfortable and eventually my room mate asked me to put on clothes. I countered by saying that what Im wearing is no different that if we were at the beach and this is what I always did prior to her visiting. Eventually, they started to spend much more time in his room with the door shut.
3. This room mate had the annoying tendency that when his gf was over, he would ignore cleaning so to spend more time with her. One time he left this huge skid mark inside the toilet, I guess from flushing a giant turd. Which is the reason we have a toilet brush, to erase the skid mark after you flush. So I went to him and asked him if he saw the huge skid mark in the toilet that his gf left? ;-) I knew it was him but needed to find a way to embarrass him into cleaning it. I also asked him about it in a tone of voice where his gf would hear it and know she got blamed. Later on I heard her giving him crap that she got blamed for his stain. Well after that, he doesn't do that anymore and eventually started cleaning up.
I could tell many stories like this but this is all I can remember now... You just have to be on top of things from the beginning.