Unlike most of other classes, users and usergroups cannot be sent to trash. They must be deleted once for all.
In that way, it's important that the Managing User Accounts documentaion page is updated with the following information (suggestion):
Removing a User Account
/!\ WARNING : delete a user account can lead to serious malfunctions.
Be cautious considering deleting a user account. In most cases, it is better to disable a user account than to delete a user account. Besides, please be aware that it won't be possible to send users to trash bin for later restore. Once you delete a user, it's wiped from the system.