Tact and professionalism are called for, otherwise a potentially harmless situation could turn into a violent one very quickly.
The following techniques will help you if you have to refuse service to anyone:
Be polite and tactful. Remember that refusing to serve can embarrass the customer and provoke an aggressive reaction.
Don’t be aggressive or confrontational. Don’t use threatening body language.
Try not to talk down to young people or make them feel silly in front of their friends. It is advisable to move the person away from the group to prevent him or her putting on a display of aggression to impress the “audience”.
Depersonalise the situation. Explain that it is not a personal refusal and stress your own legal obligations. When asking proof of age, explain that it is shop policy and requests are made whenever age is unclear.
In the case of an under-age customer, politely say: “If you are 18, I apologise, but in my opinion you do not look old enough and I regret I cannot serve you.” Don’t let yourself be drawn into a discussion about your decision.
Delay your refusal to serve. Wait until the goods have been placed on the counter and then remove them. This works well in that it prevents the customer running off with the goods when service is refused and removes a potential weapon from their hands.

