Si, se puede volver a bloquear.
Dejo el texto donde lo he leído y el link
Q: Can I re-lock the bootloader?
It IS possible to re-lock the bootloader with the the command: fastboot oem lock
You'll have to boot back into the bootloader (adb reboot bootloader) to issue the fastboot command.
Re-locking the bootloader WILL wipe data just like the unlock process. Use LG Backup app/LG Bridge or another backup method first if you want your data.
Flashing a KDZ in LGUP or LG Bridge will NOT relock the bootloader.
DO NOT re-lock the bootloader if you have a modified boot/system img... restore them to stock first or you will not be able to boot into android after the lock and need to flash a KDZ in download mode.
Fuente:
https://forum.xda-developers.com/lg-...nlock-t3614719