Following information are here for development purposes and may not reflect the current state of any Ring client.
Let's explain banned contacts with a simple scenario:
Alice and Jessica are friends, and like all good friends do, they use Ring to communicate. They are both Ring contact of each other, so Alice is a contact of Jessica and Jessica is a contact of Alice. Some day however, Jessica does something really bad to Alice and Alice doesn't want to hear from her anymore. Instead of removing Jessica from her Ring contacts -- which would still allow Jessica to send her contact requests, Alice bans Jessica.
So, what does it mean ?
In the daemon
Jessica won't be notified that she was banned by Alice.
As a banned contact of Alice, Jessica won't be allowed to contact her anymore, in any way. Text messages, voice or video calls won't be acknowledged or answered in any way. Alice won't even be aware that Jessica tried to contact her.
Banned contacts are synched across linked devices like other contacts.
In Jami clients (recommended implementation)
As long as Jessica is a banned contact, the conversation with Jessica doesn't appears in the conversations list. The conversation history, however, is not deleted. Jessica appears in Alice' account banned contact list. Alice might also find/open the conversation by performing an exact search for Jessica' username.
Alice can un-ban Jessica from the conversation or the banned contact list.
The search result and the conversation indicates Jessica' banned status. Alice must unban Jessica to send her a message, call her or otherwise interract with her.
Alice can still delete the conversation history using a Delete History button.