Emma,
If you're really concerned about someone using your wireless network, then you could enable MAC address filtering. This stops any device, apart from those that you nominate, from logging onto the network. The other thing you can do is disable the network from broadcasting its presence. (Sorry, if someone has already mentioned this earlier in the thread).
Tony
<Added>
Quite, Griff. A Media Access Control address (MAC address) is a unique identifier attached to most forms of networking equipment.
Thanks Tony, I don't think it's been mentioned and I had forgotten about it, though I'm sure I've seen those acronyms. I think I'm only as worried as it's sensible to be, if you see what I mean, but even what's sensible can be quite hard to work out.