You don’t have to level up to this, you can just say it.
I mean, you probably can’t to your boss, but you can say this to all those other emails you get where you don’t have any particular obligation.
If you think that “I don’t want to” is a bit harsh, you can be honest but still soften it. I told someone the other day, while delivering a “no”, that I’m trying to wrestle some of my personal time back. That’s true, I am. That way it prevents them from reading in between the lines that you don’t want to because you they suck or their thing sucks.
If you think that, don’t answer at all. This is email. While I think answering things is generally good and I’m broadly against bridge-burning, you don’t owe random people asking you for things responses.