What is he asking that is flirtatious?
Yes, a young, attractive, single female is going to get attention, but most times, these old guys are trying to relive their youth and console themselves with attention from a younger person.
So...if he is being offensive and scary, you can say "I will get the store manager to see if they can help you...I am not understanding what you want from me..." type of statement.
If he is making casual conversation, she can simply say, "I hope you have a really nice day; I'd love to chat but am really busy today and need to get my errands done."
Sadly, these guys seem to think it is their god-given right to at least try.......
Thoreau said, 'A man is rich in proportion to the things he can leave alone.'