Stop using Steam and get the game on GOG which offers the old-style fixed version installers.

google is your friend but ill go ahead and answer your question

go to the steam folder and open steamapps and find appmanifest_377160.acf and right click it and set it to read only

its best at this point to launch the game using the Fallout 4 script extender exe and do NOT verify the files of the game or it will force the update.

Verify game files is a complete placebo and fixes nothing so there is zero reason to do it. If you break your game with mods or encounter a bug verifying the game files will do exactly nothing to solves these problems.