This article describes how to block incoming calls based on the phone.

Polycom phones can Auto-Reject a call to Voicemail & Auto-Divert calls to an outside number by creating a Phone Book contact.
TIP: If you Auto-Divert calls backs to the Caller ID of the abusive caller, the call will disconnect.
This enables you to prevent an abusive caller from leaving voicemails.

Note: Vertical desk phones can only Auto-Reject calls to Voicemail.

Instructions for different types of phones: