Try to log in first. The presence of additional User accounts that you don't want can be dealt with later.
If you remember your short user name (if you do not, boot Single User mode to determine it using Step 5) then:
At the login screen, the one I am assuming now shows two User accounts configured by the thieves, press the key corresponding to the first letter of a User account. Which one does not matter. Just press the key and do not press Return. That will select the User account without prompting for a password (edit to add: pressing a left/right arrow key does the same thing).
Then, press the two key chord option return.
That will present two fields in which to type in your short user name and password.
Determine if you can log in using that method.
It would help if you would please provide the OS X version installed. That way, someone might be able to duplicate the problem.