Why would you want to penalize the contractor when an unknown person fiddled with the extinguisher? Just replace it and send out a warning notice that "such an act is dangerous to all working on your site, and all employees are duty-bound to inform the company of offenders as it is a matter of life and death for their own good and cannot be ignored."
In the event the extinguisher was disengaged due to an error, just ignore it. Also, ensure that when you issue such a notice and warning, you also conduct regular training/drills for all employees at least once a year on how to use these devices. Surveillance cameras are a good option to consider to avoid such issues.
Good luck.