It is possible to delete "maintenance calendar events" manually using the F&O interface via Asset management -> Maintenance schedule -> All maintenance schedule.

It is not currently possible to delete AssetMaintenanceAssetCalendars items using, say, Power Automate / Power Apps.

Deletion of AssetMaintenanceAssetCalendars items is vital for customers with thousands of assets, with thousands of maintenance events and complex workflows. Due to this missing functionality, our business stopped using AssetMaintenanceAssetCalendars and implemented a custom solution, which was bespoke and costly.