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.