Customers react to bad service in many different ways. The two main ways are to either talk to a supervisor or just leave without telling anyone. Which would you rather have happen at your organization?
If an upset customer leaves without telling anyone, you're probably not going to see that person again . . . and they're going to a few of their friends who are then going to tell a few of theirs. You'll never know what the issue was and you'll never know that it needs to be fixed.
If an upset customer stops to talk with you, sure it's taking time away from your own agenda and you have to have personal contact, but realize that they're taking a personal interest and time to let you know that you have a problem. The only way to fix the issue is to know about it, right.
If you don't have readily available means to let customer contact you, you're missing out - big time - plus you're ticking them off even more. If people want to complain, they're going to search and find a way. But, on the other hand, what about the people who want to say nice things or share ideas that could make other customers' visits better? They may take a quick look on the Internet for a contact, but if they don't find it quickly enough, you're not going to hear from them.
This is all a part of your organizations Brand.
A means of personal contact, phone numbers, and emails - ensure that they're readily available at the "point of sale", your website, and brochures. Good customer service sells.