Auto Deleting Stalker Voicemails

I have a stalker. They leave about 15 to 20 disturbing voicemails per day. 
You can block phone numbers, but you can’t block them from leaving voicemails. 
Currently, I have to go into the text to speech voicemail app to manually delete them and I cannot turn off notifications for my stalker’s voicemails. 
Please tell me there is a way to automatically delete voicemails. 
I need this phone number for work.