Because this has not happened before, your principle assumption remains that the procedures as specified normally work and are understood by everyone. Perhaps for this particular person the rules were not clear and well understood? Check out the subsitution test! In case there is no doubt, then obviously someone knew better than the system and deliberately deviated.

What are you correcting:

The person in question needs to understand how important it is that the operational practice follows the applicable rules. It is very important you do not let this opportunity slip, otherwise deviations and rule-breaking may quicky become the norm and your safety culture deteriorates. Were other workers already pointing out to their colleague he was violating the rules? You wish they had!

How are you correcting:

Assuming your procedures are in order you are definitely coaching the worker back on track of nominal behaviour. You could have him train the procedure again. Check if there is an 'attitude issue' with the person involved which could make things more difficult and could necessitate the first of correctional measures like a verbal warning.