...Possible Variables (optional) - they need export and must be stored in "/etc/default/grub":
export GRUB_DISABLE_OS_PROBER_EFI=true
-- Disable the script 30_os_prober_efi
export GRUB_DISABLE_EFI_directory_loader=true
-- Disable directory and/or loader (first found=disabled)
Example for disable the loader /EFI/Microsoft/Boot/bootmgr.efi:
export GRUB_DISABLE_EFI_MICROSOFT_BOOT_BOOTMGR.EFI=true
Example for disable the complete folder boot/efi/EFI/grub:
export GRUB_DISABLE_EFI_GRUB=true