Have everyone close the program.  Wait at least one minute after the last person logs out, then log in as sysadmin in single-user mode.  If you get in, then switch to multi-user mode and the phantom user can log back in again.

If it doesn't let you in, log in as sysadmin in multi-user mode and then close the program.  Wait the one minute and log back in as sysadmin in single-user mode.

If it still isn't working, the process gets more technical.