When deleting all user roles the user won't be able to login, because no user roles means no rights.
A user can be set inactive by going to the MxAdmin > Users form double clicking on a user and untick the checkbox with the caption 'Blocked'. However this property is reset every 10 minutes, so when the blocking is not temporary it is beter to tick the checkbox with caption 'Active'. Now the user won't be able to login until the time comes someone unticks this property.
Good luck!