chobit

Some phones has the ability to put call into a block call list.  Otherwise, another suggsestion is to add the phone number to your phonebook, then custom setup the user ringtone to slient, or in some cases, you can set it to go to your Voicemail box directly..!

I my case I have a Motorola Droid (Milestone for Telus) which support all those features.

Good luck

chobit

BTW, if this is not cell phone, then I don't have a clue.  My home phone allow me to do the same to block unwanted call online, or with my cordless phone.