You don't own the program, the company does. If they are making changes to the program by order of the management or the customer, that is not an issue.
I think this is more of an issue with your company's change request and testing protocol, not an issue with the other developers in the company. You should be complaining upwards, not down.
The only way this would properly be an issue with the other developers is if they made the changes on their own initiative with no input or permission from management. If so it would be a disciplinary matter.
Edit based on new information provided in comments:
If there are developers who are making changes to working production code without permission and then deploying this code to the production environment without permission, then this is a gross breach of trust.
If you want to handle it with kid gloves, an email to the team along the following lines should be sent:
It's come to my attention that changes are being made and deployed to
the production environment without following the company's change
request or testing protocols. This is unacceptable. All change
requests must go through the proper channels and must follow the
company's change request and testing protocols. Failure to do so will
lead to disciplinary proceedings.
If the company's change request and testing protocols exist in a document, attach them to the email. I would also recommend CCing the managers overseeing your team.