The user cannot be deleted because the user has been logged on to the system. To deactivate a user, set the user’s state to Disabled.

When you try to delete a user from BC you could encounter the following error:

The user “ADMIN” cannot be deleted because the user has been logged on to the system. To deactivate a user, set the user’s state to Disabled.

This message is somewhat confusing because this user is currently not logged in. To resolve the issue go to the User Personalization List (9173, List) and delete the mentioned user. Now you should be able to delete the useraccount.