He gets online all hours of the day and night. We want to block his WiFi access, is there a device we can put in our home that would prevent him from getting online in our home?

Take the computer away. And block interenet access on his cell phone. Worked for me!

My son may only use a computer if I'm in the room. To prevent cheating I have a laptop which goes with me when I leave the house.

I don't know anything about WiFi, but I'm sure someone here can help you from that point of view.

But he's obviously doing something that you're asking him not to do, meaning he's got an issue with listening to you. You can password the computer itself. Don't know if it's laptop or desktop, but laptop can be locked up somewhere. If he needs it for school, allow under your supervision only - i.e. during designated hours and then he gets off, no argument and no questions asked.

Take away some other privilege if he continues to do it. The problem isn't in the computer.

If it's NOT secured then he isn't hacking it, just using the freely provided service.

Anyway I digress.
PHYSICALLY remove the laptop from him, only sure fire way.
Sounds like you need to remind him of the house ground rules.

There's software for monitoring and Recording sites and setting a time limit etc.

Internet Parental Controls | Web Filter | Porn Filter

If you want to be sly, remove the wireless card in the computer.

Yes, as Curly pointed out, he's not hacking, he's using the freely open connection. So go to your neighbors and ask that they secure their network. Help them to do so if they don't know how.

But the others have a point, you have a bigger problem if your son is disobeying your instructions.

I agree with others that blocking the inappropriate use on the computer technologically is kind of a secondary issue. The bigger issue is that he's being given a privilege to use the internet when he doesn't deserve it and is not mature or responsible enough to have it. He really doesn't need it, so take it away.