As Randy Landry1 says, Google for "silent ringtone", download and install on you iPhone as a ringtone.
Now create a contact for the offending phone number. Assign the contact to the silent ringtone and set no vibrate.
When that number calls it will not ring, nor vibrate. It will light up the screen, but if you do not see that, it will not bother you. They could leave voice mail, but that is easily deleted.
And if it is a situation where the are violating the do not call list, you can use the missed call log to report the number to the FTC.
Also, if the calls are abusive or threading, and the leave a voice mail, you something to show the police.