How to fully remove Ubuntu from UEFI boot menu

Assuming you are using GNU GRUB2

There should be a file named grub.cfg under /boot/grub/ that is generated from the scripts in /etc/grub.d/ and settings from /etc/default/grub on executing the command:

update-grub

You need to login as root to do any of the following.

A quick'n dirty way is to find any occurance of the enties you want to remove.

# find occurrences in /etc/grub.d/ scripts
find /etc/grub.d/ -type f -exec grep -n 'ubuntu' {} +

Then edit the files found by removing those occurrences.

Make sure you remove the right menuentry, I recommend making backups.

After that simply execute:

grub-mkconfig -o /boot/grub/grub.cfg

OR

update-grub