Hello I am admittedly very new to Insurgency so maybe there's a setting I am missing but I don't think so.
Currently, from what I can tell When and/or If you Join a Game that has mods or maps that you don't have it will spend the time Downloading Everything In the Server Rotation instead of what is needed to get into the game Now. and down load the next map if needed.
Cancelling this Process Doesn't Actually Cancel it. The Download will Continue in the background Even If you have joined an Entirely Different Server!
Adding to this, The Download will Persist Even After the game has been Closed leaving the Process of "InsurgencyClient-Win64-Shipping.exe" running in task manager.