It is possible to go into the Modules folder, find the offending scenes and actions, and delete them. That stops them from happening.
Hard cold reality; the game doesn't do a 'harem' scenario according to your exacting specific definition. Which is not to imply your definition is wrong by any means. It's just something that's not in the game. The closest thing to it is polyamry. That's not the same thing, but it is the closest thing. If you go into the Polyamory module and arduously scan through the scenes, you will eventually find the specific ones you hate and you can then excise them. You can even find the 'organize poly party' action and delete that, and it will no longer even be on the menu.
It's not ideal for what you want, but it's what's in the game right now.
It would be possible to create custom scripts for what you want, but...not easily. The scripting in Life Play is a bit tiresome, especially when you want branching options that involve similar subroutines. You have to laboriously reprint the subroutines every time they're called, instead of being able to set it aside and just call it from where it is and return to the function that called it. I'm doing a bad job describing it, but basically it's just really cumbersome, lol.
But not impossible. Best of luck to you.