Yes & No.
Yes it's possible.....just not out of the box. You'd have to jailbreak your phone then buy an application called iBlacklist. Not something I'd recommend really unless you know what you are doing technically and it depends on whether you NEED the facility enough to pay the $12 for the app.
A bit of a clunky workaround would be to record a silent ringtone then assign it to the person(s) you don't want to hear from.