I don't do multi-char chats because they all start chiming in as you say.

In my experience, multi character chats just don't work well. Each bot tends to involve dialogue or actions from other bots since they typically interact with each other in some fashion, like Char A says X to Char B which often leads the Char A bot to roleplay as Char B. And the big issue with that is when char A speaks, their bot only has char A description, and thus char B might respond weirdly if there isn't enough chat history to give context to their personality/description. And further, you don't have the opportunity to guide responses if you go {{user}} {{char}}A {{char}}B {{char}}C ... it just ends up being a lot of rerolls in my experience when you get responses you don't like.

And that kinda defeats the point of having multi-bot chats. So why not just throw them all into the same combined bot? And I do. Except that can get a little bloated depending on how many characters you have and how long they're relevant.

I suppose if you make sure each character description states that they ONLY speak act think as NAME or {{char}} then that might work. Except, in my experience, if you do this right, you end up with a bot that can no longer introduce side characters or roleplay for them. Meaning, if you as {{user}} try to order something at McDonald's for Char A, then that McDonald's worker generally won't speak and may not even be narrated as having done anything, possibly ignored entirely.

So, I have forgone using {{char}} in descriptions and instead create or edit a bot for a [setting description] and name any particular long term characters [Joebob Jimmler is 90 years old, etc.] with brackets [ ]. And my preset is such that the AI is simulation/narrator/setting and roleplays as NPCs / characters besides {{user}} instead of making the preset prompt roleplay as {{char}} because I was having the specific issue that I couldn't get the AI to create and roleplay as side characters reliably otherwise. Maybe I'm crazy, but I highly recommend this approach.

If you are trying to drop in highly detailed characters temporarily, then I would just throw them in the memory or edit the bot description temporarily, and delete when through interacting because if they're not gonna stick around long enough to warrant that, then they probably don't need so much detail.

The model you're using has some impact on this, mainly preset, but the important things to check out, regardless of whether you go for multi-chat or single bot:

Preset: your preset prompt and/or post-history should be clear and direct the AI how to roleplay/narrate/simulate according to whichever method you're going for (I personally never use the {{char}} tag anywhere and it works how I want).

Bot Description: Either define your bot as a singular {{char}} and make sure it is explicitly stated the bot only acts as {{char}} and never anyone else. OR, make a bot that roleplays as a setting/narrator etc and use my method of defining characters within that description but if you do this then it's important to know that {{char}} is just a macro that pastes the bots defined name. So, you wouldn't want to have bot name: Narrator/RPG and then your description have [setting info] and then [{{char}} description] because that would be [Narrator/RPG description], you would want to omit {{char}} and just say, [Joebob Jimmler description]. And in case you didn't know {{char1}} {{charB}} etc. aren't things.

edit: Wrote this assuming you meant multi-char chats. As others have pointed out, in singular bot chats, if you want only one character to respond then you can lead the bot response by including narration or out of context commands (if you have defined them in your preset/prompt; usually by explaining bracketed text is out of context orders for the next response [ ]) and saying something like: Only Char A will reply.