My ex won't pick up her stuff?

Write her an email or send her a letter (so you have a papertrail) saying that you need her stuff to be gone by a certain date (make sure you give her a decent amount of time - a month would be more than enough) or you will get rid of her stuff. You aren't obliged to provide free storage for her. As long as you give her fair warning and provide a clear dedline and consequences with written proof of this you should be fine. If you're worried about it call up the local police station to double check.