it is not clear what you mean by disabling replay. When you play without FAF there will be no replay in FAF vault. When you play with FAF, replay is being send even before the game is completed so people can watch your game even before game has ended (live replay). after that replay is saved locally and on faf servers.
If you play without FAF you need to manually press "save replay" button and choose a name for replay file, otherwise replay will be written (and re-written) in "LastGame.scfareplay" file.
If you want replay to not be written at all you will need some mod... there is no built-in functionality for that.
Why you want this? think it would increase performance?
replay files are fairly small in supcom... don't think they affect performance that much...