Final answer:
The first step in stopping service to intoxicated customers is to politely inform them of the decision and explain the reasons in accordance with establishment policies and local laws.
Step-by-step explanation:
The question seems to stem from a real-world scenario related to responsible alcohol service, which might suggest it is related to a Health or Business class, particularly in the context of learning about responsible service of alcohol. However, the provided solution about classifying alcohol as primary, secondary, or tertiary refers to a Chemistry concept that deals with the structure of alcohol molecules, and is not relevant to the question asked. The first thing to do when deciding to stop service to a group of customers who appear to be intoxicated is to politely inform them of your decision and the reasons behind it, ensuring to adhere to the establishment's policies and local laws pertaining to responsible service of alcohol.