How do I prevent the Windows 10 "Maintenance Activator" from waking my PC randomly (even after disabling wake timers)?

The first step is to deactivate Wake Timers in your power saving profile. To do this, go to PowerOptions > Advanced Settings > Sleep > Allow Wake Timers > Setting: Disabled.

Next go to Control Panel > System and Security > Security and Maintenance > expand Maintenance section > click on Change maintenance settings. Uncheck the option that allows the PC to wake up. You may need to change the time first, or the option will stay unchecked but the task will remain armed. This may not happen, but it is possible.

This will disable the Maintenance Activator. If this does not work, there is another fix.

First, open Regedit. Then navigate to the following registry key: HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Schedule\Maintenance. If this doesn't exist, create it. Create a 32 bit DWORD value named MaintenanceDisabled, if it is not already there. Set it to 1. Then reboot your computer. This will disable the automatic maintenance.

This worked for me:

Configure power options > Advanced settings > Hibernate > Allow activation timers > "Setting: Disable" > Apply > OK

And that's it!