You can block any phone number by following this procedure:
- Go to your inbox and search for the number you want to block, then click on that entry to expand it
- Click on 'More' -> 'Contact'
- Click on 'Edit'
- Now, click on '+ Choose Groups'
- Then, apply the Blocked tag so this number gets blocked. Click on Back when finished (do NOT click on Close, otherwise the tag won't get saved).
- Finally, you will see that the Contact is assigned to the 'Blocked' group, and this number won't be able to call your numbers anymore. Click on 'Save' to apply the changes.