Turn off notifications for a specific chat by muting it. Click the three dot menu on any chat and then click Mute (Figure 1):
Figure 1 Mute
You will be able to identify that the chat has been muted by the strikethrough bell icon (Figure 2):
Figure 2 Muted chat
To unmute a chat, click the three dot menu on a muted chat and then click Unmute (Figure 3):
Figure 3 Unmute