By being assertive.
Which is easier said than done, especially if you haven’t had any training in being assertive and have feelings invested.
You need to figure out what terms are non-negotiable for you. If you don’t want to work on the project anymore, then you keep telling them that you don’t want to work on the project anymore in a calm and polite manner while you acknowledge their frustrations but keep your ground.
If you can be pursauded to work on the project for money, then you negotiate this opening, and simply keep saying that you can’t work on the project for free, but that you’re willing to keep working in it, if you get paid.
You need to keep things relevant though. The fact that ideas are cheap, isn’t relevant to you not wanting to work on the project, and I promise you that you’ll fuel the fire if you bring that up.
