Glad to be of help SM! Our AP's room is basically a studio away from the main house and I've always let bf's/friends stay (even Mum & sister at the same time). We are really close to London so I find that, although they stay, they are never here. In fact, once dh took ap's bf out for a drink and got him totally drunk.
The way I see it, an Au Pair IS part of the family and they are my responsiblity whilst they are here. In fact, now she has gone, my dds want to go on-line every moment to chat to the ap's bf as they got on so well with him (and her).
This has always worked well, until the last few experiences. ExAP asked if she could have a friend to stay and we agreed (she has sofa-bed in her room). But they drank my wine and didn't even replace it/mention it. Not a problem ordinarily as I can't drink at the moment and I saw she had brought some food left over from their breakfast which I thought I would have the following morning. When I went to the kitchen cupboard in the morning, she had removed all the food she'd brought and hid it in her room. Dh thought I was over-reacting a bit and so I didn't say anything.
The next time she asked if her friend could stay, I said yes and it turned out it was 2 large men. Dh came home from work to be confronted by them, sitting in HIS chair, with a can of beer in their hands and not even offering him one. He was fuming.
So we've agreed that, IF we ever get another AP (I'm on maternity leave at the moment) we will remove the sofa-bed from the room so that they can only even have 1 visitor. If they do want someone to stay, we will want to know who they are and set some ground rules (re. food/drink/etc).
I think that once you start off being generous you create a rod for your own back and they take advantageous of your good nature.
So I think the hostel is the best solution.