Wireless internet randomly switches to the neighbors

We have had our wireless internet set up now for nearly 3 years without any problems. About two months ago I noticed that a couple of different people were using our connection and they were taking up quite a bit of bandwidth usage which made our connection slow. So, I decided to use a WEP key and this solved that issue. The problem came when our neighbor, who had previously been using our connection, installed his own wireless conection and my computer now randomly switches to his connection even though his is locked. It connections, but has the little exclamation mark that says limited connectivity. It is so frustrating because when I go into the Network properties it only shows my connection as preferred. How can I stop my computer from switching to his connection?