Thomas's answer is excellent and very true - I think of it like a *wish* spell in that if you're not going to go with one of the pre-fabricated options, the result is entirely in the GM's hand.

To address your other question "what would be a better word to use to make someone drop concentration using Command?", personally I like *"neglect"*. Of course, it's still very much up to the DM *what* they neglect, but I think it's just slightly less ambiguous than *stop*.

As a side note, a mechanical reason for the GM to deny a quick and easy way to force the enemy to lose concentration is that *command* is a 1st level spell and would enable what is clearly a very effective strategy. The closest examples to this are spells like *sleet storm*, *earthquake*, and *storm of vengeance* (3rd, 8th, and 9th levels respectively) which [interfere with concentration outside of just dealing damage][1], and that's before considering spells like *acid arrow* (2nd level) which was created specifically as a way to deal continuous damage [to increase the odds of disrupting concentration][2]. Moreover, the most game-changing original spell in the OGL-based video game Solasta: Crown of the Magister is ["Swarm of Bees"][3], a cantrip specifically meant to disrupt concentration. There's a reason WotC have never included a similar cantrip into official material, despite how much fun it would be to pelt the evil wizard with bee-hives.

  [1]: https://rpg.stackexchange.com/questions/74432/in-what-ways-can-a-spellcasters-concentration-be-disrupted
  [2]: https://www.belloflostsouls.net/2019/03/dd-a-brief-history-of-melf.html
  [3]: https://www.youtube.com/watch?v=7blp_fW64QE