Linksys Ethernet (local network) problem

[Mockery]

We've got a linksys wireless ethernet installed here. Right now it's just myself and Re's computer on the network... but recently there have been times when another person's computer has appeared on the network. I was able to get in and view his files 'n everything.

Now I thought this shit wasn't supposed to be able to happen, but sure enough, it's happening. I don't have anything secret on the network... I never keep important files of any kind on there. Still, I want this guy OFF the network permanently.

Any suggestions on how to do so?