HTC one...
Tap the Applications icon from the Home screen.
Choose Contacts then tap on the Menu icon.
Tap Manage Contacts, then on Blocked Contacts.
Now tap on Add icon and select from contacts you’ve already saved in your phone or simply add a number.
To select from People, check the box next to the contacts to block and tap Save.
To add a number, just key in the phone number of the person you want to block and tap OK.