You might try spraying with an essential oil spray. Geranium is one known to keep away mosquitoes, fleas, etc. Info here:
You could also use a plug-in atomizer if you've got a plug beneath or beside the window. The scent should repel them from wanting to come in. I use AromaBalls around our house for various tasks that I use essential oils for:
I love all the options of what EO's can do. You can find more things to use them for here:
We've been able to eliminate chemical products from our home by using essential oils (EOs). They are so antimicrobial that I make all my cleaners with them and Sunshine Concentrate:
Hope that helps. We had no seeum's in Alabama and they can be a real pest when we were gardening. We used EOs in aloe vera on our skin to keep our sanity.