The only way to block an update for Civ VI for an indefinite period of time is, as Leyrann says, to put Steam into offline mode.
Note that this is not that same as opening up the friends window and toggling it to"offline" - that only an appearance based thing and wont stop updates. Instead, make sure you go to Steam in the top left corner, and select "Go Offline..." which should be the second option down.
From this point on, when you start Steam you will be prompted whether you want to continue playing offline or not - obviously you keep picking play offline until you change your mind
There are a few other things to keep in mind however. Turning off automatic updates
is not a solution. Lots of people fall into this trap because the settings are kind of vaguely worded. Turning off automatic updates does not stop Steam from queuing the update in your Download List, it just stops the patch from being downloaded without an active input from you. What this means is that
the first time you start the game after the patch is released it will begin downloading/installing and you will be back to square one again.
The
only way to stop Civ VI from updating is to stay in Offline Mode for as long as you want to play without the patch. This has the added side effect of preventing you from updating
any of your other games on Steam as the moment you go online to update one of them, Civ VI will queue its patch and you will have to install it before you can play again
regardless of whether automatic updates is toggled on or off.
Obviously if you don't have many other games on Steam or don't care about keeping them updated, this isn't a problem