One way to report and enforce unauthorized sellers on Amazon is by signing up for Amazon Brand Registry. While not specifically made for MAP policy enforcement, it can still help a lot.
This program allows brands to access advanced reporting tools, have more control over their brand product listings across the entire platform, and protect their trademark.
With Amazon Brand Registry, you can restrict the total number of authorized sellers that can sell your products on the eCommerce giant and even detect any trademark infringements by resellers.
While Amazon does not actively protect your trademark, registered brands are usually more protected when it comes to trademark infringements and unauthorized sellers.