Mute single speaker?
Simple method, as already stated, System Prefs > Sound > Output. Set balance slider to the right.

More comprehensive method, in case you are listening to stereo &amp; need the information otherwise missing from the other speaker - you also need…

System Prefs > Accessibility > Audio  

check 'Play stereo audio as mono'

This will sum the left &amp; right channels so you will hear both from the one speaker after you balance across to it.