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, 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. Moreover, the most game-changing original spell in the OGL-based video game Solasta: Crown of the Magister is "Swarm of Bees", 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.
