It's a common question. Here are some ideas about what you can say when you have the same customer who repeatedly comes in and asks for a discount.
First, Don't Get Frustrated
No doubt it's frustrating to be asked each time, and each time having to refuse, but this is a situation where you have a repeat, perhaps even loyal customers. So, you don't want to lose the repeat business, even if you are getting tired of answering the same question.
Focus On WHAT YOU CAN PROVIDE
They key here is to offer something you can provide that might be of value to that customer. Even if it's not what the customer is asking for, you show you are trying, and that's important. Of course what you can offer depends on what is practical. That said, here are some examples:
Retail Environment - Refusing A Discount
"I CAN offer a way to save some money, though, even if I can't discount this particular product. Have you considered purchasing our "noname" brand for the same thing. Our brand is made by the same manufacturers, and you could actually save between 10 and 20 percent.
Business To Business - Saying No.
"Perhaps you didn't know that by buying more than five units of the same thing at a time, you'd be eligible for a ten percent discount anyway, and you'd not run out so quickly. Did you want to add more to your order?"
No Discounts Possible? No Problem
You don't have to offer just a discount. You could offer a corporate loyalty card offer, or something else that could be valuable. For example, how about some time saving tip for the shopper/buyer?
"I understand price is important, but you might also want to take advantage of a new service we offer, so that you can order things online. You'd probably save tons of time."
What's Important - How You Say No
Notice that in our examples, we are shifting attention away from the discount issue. We deal with the discount refusal AFTER we offer something different. That's important. That's why we don't start off the sentence/paragraph by saying "No, I can't do that". Offer something first. Then do the refusal as a kind of "after thought".