I recently bought the whole PC series of Saints Row and after I began the third, after I leveled up, I saw that most of the power upgrades were already done for me and many clothing items were already bought.
I know the game is quite easy economically but I want to start a game in which I can actually level up, not sideways.
Is it possible to begin a game in which (preferably) everything is locked?
I probably have all the downloadables of the game.
Right click on "Saints Rows: The Third" in your list of games in your library and select "Properties". Click on the on the DLC tab and search for the entries called "Saints Row: The Third - Unlockable Pack" and "Saints Row: The Third - Bloodsucker Pack". Uncheck the boxes next to them and these DLCs will no longer be active. Unfortunately you may have to start a new character as your existing character already has everything unlocked.
Unfortunately I don't have the "Saints Row: The Third - Unlockable Pack", but here's what the DLC list looks like:
Note that many clothing items and certain weapons and vehicles that are enabled by the other DLCs will remain. You can also disable these DLCs as well, but there's no way to unlock their content in the game. If you want to use them you'll need to have the DLC active. Since you can simply not use any of these thing if you think they make the game unfun I would just leave all the other DLCs entries checked.
