Organizations should be very thankful to anybody who has made the effort to report an accessibility problem!
This is a luxury!
You get a chance to improve, reducing the number of visitors/customers who need to go to a competitor, or become unhappy and complain about you to their peers.
Zoe Portlock says it's "absolute gold dust", and I agree.
Even if it can hurt a bit to learn about our own problems, don't shoot the messengers! They're your best friends! Say thanks, try to help them, and forward the information to somebody who can fix the problem. As Zoe says: Don't be too formal in your response. There's a person behind that message! Possibly a person suffering from your mistake.
Offer a "barrier bounty program"?
For security problems, there's something called "bug bounty programs", offering rewards to those who find and responsively report security problems (instead of exploiting them). Maybe consider something similar for accessibility problems?
But if you do, be careful. Only offer rewards for reports of serious problems that affect real people. Otherwise this could result in useless reports from superficial tests - and sales calls. Thanks to Mia Ahlgren for the caution.
Even better than having barriers reported, of course, is to have no barriers at all. We like to recommend the "butter spread method for accessibility", where accessibility and user involvement is considered throughout all phases of a product or service lifecycle.