To prevent unwanted contact from another user, you can use the Block option. This will disable the user from being able to call or send you messages.
To block a contact:
Find the profile page of the contact
Tap on the 3 Dot icon in the upper right corner of the screen
Select Block this person.
You can also Report inappropriate images/contacts from this page which will automatically disable any communication.
To unblock a contact:
Go to your profile page
Tap on the Cogwheel icon
Tap on Privacy
Tap on Block List
Find the contact you want to unblock and tap Blocking
Confirm you want to unblock