@beggi321 I think @caver01 is right and you didn't actually enable dispmanx. You visited the menu from the Configuration section, but that menu shows something only if you have an emulator that supports dispmanx. If you don't have one installed, then the menu just flashes by and doesn't perform any configuration.
If you didn't do the steps that @caver01 described earlier - and set dispmanx as the video driver for RetroArch - then you didn't actually enabled anything, so there's nothing to disable.
Is there a reason you believe that dispmanx is enabled in your system ?