Phone Numbers to be ignored

Solution 1:

Without jailbreaking, your only option is the network operator's (AT&T, Verizon, etc) blacklist option. There may be a monthly fee, $5/mo on AT&T for example, but will allow you block receiving calls from any number you like. iBlacklist will work if your phone is jailbroken, but it costs $11.99 - a little high in my opinion.

Solution 2:

You can always make a silent ring tone as your default ring tone. Be sure to assign ring tones to your contacts that you want to hear from.

Solution 3:

I was wrong! My other answer was incorrect and has been deleted. There's an app in the App Store called "Blacklist" that looks to do exactly what you want. I missed it before because I'm not in the US and it isn't available in my country, but your profile says you're in Philly, so you should be golden. $1.99.

http://itunes.apple.com/us/app/blacklist/id310726855?mt=8