"Debugging and troubleshooting", "List of MyChat Client errors", "Error #0310".
 
Error

Error #0310: the user with UIN XXX is already blocked, dismissal is impossible
 

Error description

You can't fire a user because his account is already blocked on the server.
 
Problem solution

The dismissal procedure is complex. Besides account blocking there are other operations performed.
 

However, an administrator can block an account as a temporary solution and then it would be impossible to perform the dismissal procedure, for example, by mistake.