Thanks for all of the advice! I'm a fan of the air conditioner idea, but my man rolled his eyes at me.
He ended up recruiting a carpenter-friend of his to make a screen that has a wood frame which can be used to keep the window open, rather than being in front of the window. (It's a guillotine style window). Hopefully that will be the end of it. Fortunately it's been chilly lately (did I seriously just say that cold weather is fortunate?!?!) so the window's been closed, and we haven't had any visitors. But I still see the cat outside at all hours... I'm a tad concerned. The last time he was here he left a poop that was covered in mucousy blood. We left a note for the neighbours noting our concern, but it doesn't look like she's done much.:sad: