You can block any user on your Telephone's contacts group with whom you do not share any other group so they will not be able to communicate with you.
On the main menu, choose Settings and go to the Blocked option.
Press the + icon on the upper right corner to add users to this black list.
Choose the contact you want to add to the black list and confirm selecting Block.
To unblock a contact, push on the bin icon just next to the contact name on Android or select the red - icon on iOS.