Open terminal (Alt+Ctrl+T) and follow the below procedure :
Paste the following command in terminal sudo gedit /etc/default/grub.
It will open a file named /boot/grub/grub.cfg file.
Add GRUB_DISABLE_OS_PROBER=true at the bottom of this file.
It will prevent Windows being added to your grub menu.
Now to write the change, run sudo update-grub
You can then run cat /boot/grub/grub.cfg to check that your Windows entry has disappeared.
Restart your device to check the same.
