Turn off notifications for a specific chat by muting it. Select the three dot menu on any chat in your chat list and then select Mute (Figure 1):
Figure 1 Mute
Note: You cannot mute or unmute a chat from within the chat, only from the chat list,
Muted chats will have a strikethrough bell icon in your chat list and on the chat itself (Figure 2 and 3):
Figure 2 Muted chat in chat list
Figure 3 Muted chat in chat module
To unmute a chat, select the three dot menu on a muted chat in your chat list and select Unmute (Figure 4):
Figure 4 Unmute