Unfortunately cell phones cannot block calls from anonymous callers. Your best bet is to kick all blocked calls to voicemail & pretty soon they will get tired of calling. If you have M2M, it will not cost you any minutes to check the voicemail. If it persists to a greater extent, contact the police & file a report. The police should be able to obtain the caller's phone number by submitting an affidavit to Cingular.
I hope this helps.