I use Trello integrated with Slack. I am trying to remove a team member from one of my boards as he is no longer working on that account. I have removed the team member from the board by converting it to Private and have also removed the Slack power-up for that board. However, the team member continues to get notifications of updates to the board on his slack channel. How do I stop this? The team member continues to be on two other boards on my Trello account.