The common saying in business is the customer is always right and of course we always want to do everything in our power to comply with their requests, but you absolutely can tell a customer no when the situation calls for it.
Unfortunately, the word NO has become a bad word in our society. It's not polite to say no to people when they ask for something and it's to the point where you seem aggressive when you do so. But this doesn't mean you shouldn't.
NO is one of the most powerful words in the English language and people need to reclaim it.
When we hear the word NO, we automatically feel rejected or feel as if the we've been denied some type of right. But in most cases in the business world, this isn't true.
Usually the word NO is the answer to unreasonable requests that aren't feasible for the business. And if you own a business and receive a request that you feel you can't fulfill to the best of your capabilities, then it's better to say no than to do it wrong and risk your reputation.
So don't be afraid to say no to customers when you feel it's the right thing to do.