I recently learned a formula from my son's playgroup teacher that has made boundary-setting much more palatable because it makes the whole thing feel less like a high-stakes endeavour. It goes like this:

"I know you want <something> but <rule-that-comes-first>."

As it relates to playgroup: "Timmy (not my son's name, btw), I know you want to jump around right now, but there's a rule that we don't jump during circle time. If you want to jump, that's fine, but you have to go outside of the circle to do it."

Simple and effective, and not personal.

I had an adult version that happened this morning, where I said: "The opportunity sounds great but I have an investment policy that I check in with references first before I hand over those kinds of documents."

It wasn't personal, it wasn't an attack, it was just a <rule-that-comes-first>.