Apple - 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 & 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 & right channels so you will hear both from the one speaker after you balance across to it.

Open System Preferences > Sound > Output look for the balance slider and adjust as needed.