Send her intimidating letters.

What will she think if she starts receiving letters like:
You have 5 days left
You have 4 days left
You have 3 days left
You have 2 days left.
You have 1 day left as a single woman. Will you marry me?
She will probably panic, try to calm her down; tell her it is nothing, maybe a silly joke…

Do not let her call the police!