Whether you found yourself in a modded lobby getting everything unlocked, or you've improved and want a better K/D, here's a guide on how to reset your account.

First you wanna disable the cloud sync for MW2. You can do this by right clicking on the game in your library, selecting properties, and under the tab "Updates" unticking the box that says "Enable Steam Cloud synchronization for Call of Duty: Modern Warfare 2 - Multiplayer.

Now, you need to delete the STAT files. This can be accessed from the "players" folder, located in the MW2 directory. The path is C:\Program Files (x86)\Steam\steamapps\common\Call of Duty Modern Warfare 2. Alternatively, you can access it from rightclicking on your game in your library, and selecing "Browse Local Files".
Then, you want to open the folder named "players"
and delete every file with the type "STAT".

Now you need to delete the folders inside "userdata". The path should look like this C:\Program Files (x86)\Steam\userdata

Alright thats it, now you've completely reset your account and able to start fresh. Hope this guide helped becuase its real annoying to have to deal with modded lobbies. Enjoy :)