1. Navigate to Steam\steamapps\common\Batman2\BmGame\Movies
  2. Rename bac_logo.usm for the logo, startup.swf for the DC/Rocksteady intro, and startupnv.swf for the nVidia intro. This will cause them to not be recognized by the game, and will not be played.
  3. Enjoy.
  4. If you want them back, rename them to their original name.