An even better idea: disconnect the antenna. Most of the noise comes from the front end amplifier, not the galactic and cosmic background, at least in your average consumer grade receiver. And this is a quantum process that someone else definitely can't predict or copy.
This is a bad idea, the computer it self will generate clocking noise which will appear in the noise and destroy the randomness. The standard, and even most high-end, recievers don't have the shielding to prevent this sort of intrussion. Heck, that digital clock on your desk (and possibly your wrist if close enough) will cause problems as well.
And if that doesn't work, crawl up the spectrum a bit. The higher in frequency you go, the more thermal noise you'll see.
Only up to a point. Past a certain point and the processes will start to roll off their energy production.