When a Flow run is assigned to an individual user during the execution of a UI node, it automatically becomes a task for this user.
User gets notified and the task will appear in their personal task list.
Canceling a task
To do this select the Task/Run and click on the "Cancel" button at the top right.
User
A user can cancel tasks on his own from the Tasks page. This is only possible when the user initiated the task and nobody else participated in the Flow, no other users filled UIs.