Hi K.Lannoo, I'm Jayzar and I am happy to offer help.
Sorry to know that the user still receiving notifications about task that has been done even though he's no longer a member of the team. Let me help you out!
Can you ask you're colleague to perform steps below:
- Open Teams app in the desktop
- Select the Profile picture on top right side and select "Settings"
- Under Settings, select the "Notifications" tab
- Below the "Meetings", he can select "Mute until I join or send a message"
- He can then hide the tab afterwards.
I hope this information helps. If you have any questions, please let me know and I'll be glad to assist you further.
Warm regards,
Jayzar