You should be able to remove them with efibootmgr, list all entries with:
> efibootmgr -u
BootOrder: 0001,0003,0000,0004
Boot0000* Windows Boot Manager
Boot0001* Fedora
Boot0003* opensuse-secureboot
Boot0004* UEFI:CD/DVD Drive
(abbreviated output)
Note the number next to “Boot” of the entry you want to delete and then execute this command:
sudo efibootmgr -b 0000 -B
In this example, this would delete the “Windows Boot Manager”.