If they are web-based calls, block their IP (although they could use a VPN to keep generating new IP addresses). Other than that, you could tell the agent to end the call if it's from (a) certain phone number(s). If they want to be creative/resourceful, though, then you won't be able to fully prevent that. Not much you can do beyond blacklisting phone numbers and IP. At some point we might be able to take vocal fingerprints (voiceprint/vocal biometrics), and do things based on those, but that doesn't exist yet, and when they do, they will likely be susceptible to deep-fakes. You might be able to do something where it hangs up on someone if they have called more than X amount of times within Y timeframe.