Users can be active or inactive. Only active users can login to the system.

Inactive users cannot login to the system or have tasks assigned to them.

Setting a user to inactive status is the closest you can come to deleting the user. There is currently no way to delete users from Willow, as this would not only destroy historical information, but it could introduce data anomalies such as orphaned tasks. You can always filter out inactive users from the Browse Users screen so that you aren't bothered by them.