1.) Right click L4D2 on your library.
Then click "Manage" and go to "Browse local files"


2.) Go to this directory
E:\Steam\steamapps\common\Left 4 Dead 2\left4dead2\cfg\addonconfig.cfg


3.) Open the addonconfig.cfg using notepad or any application that can change notes.


4.) Change everything to 0 (The default number when you open this for the first time is 1, which disables mods on versus modes)