How to disable subtitles?

Try hitting the v on your keyboard.

There are also CLI options -nosub (for disabling subtitles that come within an .mkv, for example) and -noautosub (for disabling external subtitles from a separate file).

So, to run a video without any subtitles, you can do the following:

mplayer -noautosub -nosub filename.mkv

Depends on the video:

1. Some videos have subtitles hard-coded in (i.e. overlayed on the picture, won't remove).
2. Others have a separate subtitle track (external file)...
3. Others still may have a track encoded in it that have them.

But v is the keyboard toggle during playback. Types 2/3 this should work for - type 1 you are "SOL".