Stop Media Playback through adb
Use KeyEvent to do the job.

adb shell input keyevent 85


85 corresponds to KEYCODE_MEDIA_PLAY_PAUSE. More keycodes can be found  here or here.

Alternatively, if the activity featuring Play/Pause options is in foreground then you can do:

adb shell input touchscreen tap &lt;X&gt; &lt;Y&gt;


where &lt;X&gt; and &lt;Y&gt; are the coordinates of the tap that you would perform for Play/Pause using your fingers. See my answer here to know how to obtain them (step 1 and 2 under Instructions for Tasker and Xposed Additions).

Or, you can opt to completely kill the app using:

adb shell am force-stop &lt;PKG_NAME&gt;


Edit

Kitkat and above has the command media whose usage goes as,

adb shell media dispatch KEY


KEY can be play, pause, play-pause, mute, headsethook, stop, next, previous, rewind, record, fast-forword. (Source)