Cannot delete wireless network

Hi there, was just wondering if anyone could help with this...can't find any existing topics which seem to address this!

I have an old wireless network from a shared house I used to live in which will not seem to go away! I have deleted it, moved it down to the bottom of my wireless networks list, selected manually connect instead of automatically and tried changing its name....whatever I do it seems to come back to the top of the list and it always changes back to automatically connect. The effect this has is that my current wireless network gets continually interrupted by the old one trying to connect, despite the fact that it doesn't even exist anymore!

Any help would be greatly appreciated.

Sarah