There are a couple ways of doing this:
1.) Spawn a bunch of props and use the remover tool on the tool gun to remove them. Preferably, spam a bunch and then remove them all using the remover tool.
2.) Go to the steam workshop and look for dupes that are already made for this achievement. The only downside to this is if you have a low-end computer. It could have too much props for your computer to handle, but it doesn't hurt to try. After finding one, subscribe/install it and then use your removal tool on them.
BUT ONE MAIN THING:
It HAS to be the remover tool, not Z or the "Clean level up" button (forgot what it actually says) in the utilities menu. It has to be the remover tool. I also highly suggest doing this in single-player, as you would most likely be kicked/banned from a multi-player server for spamming props. Plus, dupes are usually disabled on servers.