How to remove or delete a task
When you first create a task in Slack, a drop-down message menu is available underneath the task.
Click the ellipses (three dots) and then select "Remove."

If the task was created a while ago, you can still delete the task.
Use the /todo list command to bring up the list of tasks in the channel, then click on the ellipses (three dots) to the right of the task that you'd like to delete, then select "Remove."
