Is there a way to prevent my Mac from grabbing a different Wifi signal?

I recently got FIOS and I love it. My router is in my basement because I wanted to hardwire it into my ps3 and 360. Regardless, the WiFi signal is great throughout my house. For some reason, my Macbook keeps grabbing my neighbors unprotected WiFi signal. I guess on the 2nd floor of my house, his signal is stronger than mine.

It takes 2 seconds to switch it back, but it seems to grab it whenever I wake it from sleep and at other random times. I was thinking of telling my neighbor that he needs to protect his signal, but even if I dont, I want a way to prevent my mac from locking onto the wrong signal.

Is there a way to do this?

Running Snow Leopard.