You can turn off notifications for a specific chat by muting it. Muted chats are indicated by a notifications off icon in your chat list. Once you've muted a chat, you'll continue to receive messages, but you won't receive notifications.
You can also unmute a previously muted chat if you change your mind.
Muting or unmuting a chat
To mute a chat:
On the Chats page, open the desired chat conversation.
Select the actions icon (•••) in the right pane, then select Mute.
In the Mute notifications confirmation, select Mute. You'll no longer receive any alerts for new messages.
To unmute a chat:
Go to the desired chat, then select the actions icon (•••).
Select Unmute.
In the Unmute notifications confirmation, select Unmute. The chat is unmuted, and you start receiving alerts again.