Don't look for a word, look for an agreement with your DM
But I'm wondering whether the word "stop" would be clear enough, by RAW.
Additionally, if possible, what would be a better word to use to make someone drop concentration using Command?
But Command says (emphasis mine):
You speak a one-word command to a creature you can see within range...You might issue a command other than one described here. If you do so, the GM determines how the target behaves.
You may be under the impression that the 'better' your word is, the more likely your DM will be to rule that it 'works', and if only you can find the 'perfect word', then the NPC would have to break their Concentration by following the Command. But that's not how the spell works; the appropriateness of the word is not relevant (you might say the word itself is fluff, not crunch).
Consider the "typical commands and their effects" in the Command spell - Approach, Drop, Flee, Grovel, Halt. Note that none of them have to have the result they do under RAW based on the nature of the word itself; rather, the game designers decided on an outcome that was appropriate for the power level of the spell and then used them as model examples for DM's and players. Suppose one of these specific words had not been included in the spell description, and a player was presenting it as a novel word "other than the one described", but the DM considered the suggested effect too powerful:
Player: "I'd like to use Approach to have the target move toward me by the shortest and most direct route, and end its turn if it moves within 5 feet of me."
DM: "Command allows a single word only - what you are describing is "Approach (me)". How does the target know who or what to approach? And why do they end their turn? They have only used movement and have all their other turn options."
Player: "I'd like to use Drop to have the target drop whatever it is holding and then end its turn."
DM: "Command allows a single word only - what you are describing is "Drop (things held)". How does the target know not to "Drop (self)" and fall prone with their items still in hand? And why do they end their turn? Dropping things is a free action - what you are describing is more like 'Drop and Halt' put together."
Player: "I'd like to use Flee to make the target spend its turn moving away from me by the fastest available means."
DM: "Command allows a single word only - what you are describing is "Flee (from me)". How does the target know not to Flee from one of your teammates, or someone in their own party, or their home?"
My point here is that none of these words inherently force the condition on the target by the nature of the word itself. Instead, they were considered appropriate effects and then the word was chosen in the rules as representative.
As a further consideration of how your well-intentioned search for a 'better word' is actually misguided, consider the difference between English and the in-game language. In English, there is not a great single word for the command 'Stop concentrating!' In Spanish, however, I can say Desconcentrense! and it is quite clear. The word actually used by your caster is not in English or Spanish however - it might be in Common, or Gnome, or Draconic. Under Rule 1 ('the DM describes the environment'), it is the role of the DM to decide the vocabulary of these imagined languages within their campaign world, and also to decide whether or not there is an appropriate word. There might not be one at all; there might be one that not only works, but furthermore unambiguously distinguishes between simple mental concentration and concentrating on actual spells. So, even if you could find a 'single word' in English to use as a Command for someone to drop concentration, the DM is well within their purview to say that such a concept does not exist as a single word within the language the caster is actually using in the game (and conversely, even if there is not a single word in English for the effect you desire, there is nothing to stop the DM from ruling that your caster's language does contain such a word).
So, rather than looking for a better word, first go to your DM and ask whether it would be acceptable to have a successful Command result in the effect you want, failed concentration.
If your DM does approve this effect, then work with them to choose any word that evokes that feeling, but realize that the word itself is just for immersion and descriptive flavor, not inherent power or to fulfil some rules requirement. I would suggest words like Disregard, Neglect, Detach, Overlook, Disconnect, Ignore, Unheed (or Misheed). The DM might even say, "Okay, in play your command will be 'Stop concentrating!' but don't worry that it is two words in English; it is only one word in Common!"
A note on power level
As Gabe notes in their answer, sleet storm at 3rd level is the lowest-level official spell that provokes concentration loss independent of damage, so your DM might be hesitant to allow a first level spell to disrupt concentration on a single failed save. In such a case, it is worth pointing out (as this answer in the Q&A linked by Gabe does) that the PHB core first level spell Hideous Laughter, on a single failed Wisdom save, results in the target being prone and incapacitated, and one of the effects of the incapacitated condition is automatic loss of concentration (even though the condition description does not mention that). Now, Hideous Laughter and Command are on the spell lists of different classes, so it is not a direct comparison, but at first level Hideous Laughter with a single failed save imposes both the prone condition and the incapacitated condition (including loss of concentration), and both for potentially multiple rounds. Compared to that, allowing Command to result in a next-round loss of concentration with the same Save is hardly overpowered or unwarranted.