Yes, I unlocked my Nexus 7 2013 when I wanted to use a factory image to install 4.4 (didn’t want to to wait for OTA).

But, now I’ve decided to relock it since I rather wait for OTA (being lazy).

This I do on a windows 8.1 computer, in a command line.

First thing to do, connect it to a computer with USB.
I check with “adb devices” that it’s connected.

adb devices

Then I restart it to bootloader.

adb reboot bootloader

Which reboots it into bootloader mode, wait a while for the drivers and everything to load if you haven’t connected it in a while.

Then relock with

fastboot oem lock

And, it’s relocked! It’s really that simple. I restart it back to normal with

fastboot reboot