No, there should not have been a slap. How could a violent act improve a situation which was murky at best? How could that help the shopper to maintain anonymity? If this shopper intended to be not memorable, they probably did the best thing in the circumstances. They did not fuss about this on-site or call additional attention to themselves or their interaction. They expressed about the matter here, where we discuss all kinds of things from all kinds of perspectives.
Nothing in the opening post suggests sexual or even flirty intention. The shopper might have felt like a kid, or uncomfortable, when the slightly older person touched their hair, but there probably was nothing more than an attempt to be able to see the customer (aka shopper) and then commune with them.
Would a manager be able to sort this out to anyone's satisfaction? After all, we cannot get to heart of the matter, which is the person's intention. We do not know them. We do not know if this is something that distinguishes the person in a good or bad way among others. We do not know if this is a one-off with a shaggy dog sort who is hiding their face. We do not know if the person has a hair fetish. We do not know if the person was assuming that hair clinging to eyeballs was uncomfortable, a visual liability, or simply unhygienic. (Formal and informal parents and teachers might be the most likely persons to respond in this way to a perceived risk.) Was the person attempting to prevent an incident on the premises? We do not know anything! Well, we do know that some people are horrified. But does that horror reflect the intention of the person who made a physical contact? Does it mean that the person who made a physical contact is a harmful sort? We do not know.
Tangent alert. Once, I observed a group of persons getting their hair done at a salon. Their supervisor was touching their hair lightly, apparently in an effort to inspire the people to touch their own hair. After awhile, one of the people began to stroke and experience their own hair. The others then made contact with their own hair. This was a giant step forward for them, in that they made some progress toward making contact with other aspect of their selves. It isn't always bad to touch hair. Sometimes, it helps people. But back to your regularly scheduled thread.
@OP: Did they just brush the errant hair away from your face? I mean, they did not alter your hair, right? They did not comment on length or color, or cut your hair, did they?
Nature does not hurry, yet everything is accomplished. - Lao-Tzu