I would be careful about advice from court clerks. While the advice about a verbal notice may be correct, I would STILL put it in writing. It doesn't hurt and you have back up plus it may show her you really are serious.. But I strenuously disagree with the advice on changing the locks. This could open you up for a lawsuit for unlawful eviction. I am very surprised they gave you that advice which calls into question any other advice you got from them.
Do give her written notice and if she still won't leave then file for the retainer warrant as soon as your notice period expires.