What kind of device do they have? If it’s an Apple phone you can turn that off so the phone uses it’s real MAC and not generate one automatically for that network.

Depending on their device such as a laptop or computer they’re probably using Linux and a cli tool called macchanger you’d been to block that tool on their end or set up a whitelist in the router of known real MAC addresses. I’m pretty sure that’s the feature you want to request but that’s the term so people under stand better “add whitelisting for macs dresses”