My neighbor is stealing my wireless signal?

I have 3 computers in my home and a VOIP system tied into my wireless. My neighbors come home and tear down my bandwith so bad, I can't talk on the phone. How do I make my wireless a secure connection.
Update: MANY MANY MANY THANKS!!!!! I DID IT!,
I ALSO NAMED IT: KELLY'S..NOT4U, I COULDN'T HAVE DONE IT WITHOUT EVERYONES HELP
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