You can not use that steam account for MW2 again, you gonna have to create a new one. And if you are afraid there might be some hack files in the MW2 folders, i would suggest uninstall the game completley and then reinstall it from your new steam account.
Also, in the future, don't put .dll files directley in the MW2 folders. Because then there is a 99% chance of getting banned. Use an injector instead.