The reason why this is difficult is that you're being too British about it - and the proper British response is to avoid confrontation at all costs. So, tell a white lie and pretend that your spare room is occupied so she can't come round. Whenever she gets in touch keep coming up with white lies over the next decade or so and eventually she'll get the message. Completely British and relatively pain free for both parties.
If, however, you are feeling more continental than you usually do, tell her what you really think. This has been done to me by Johnny Foreigner - it was agonising at the time, but both parties never spoke to each other again.