When you don't have boundaries, you are constantly expecting to be transgressed upon, so half the time you let people do it and other half of the time you tell them to fuck off when it would have been fine to help them. That's what I mean by asshole.

What you're talking about is a healthy no. And I agree there are good ways and bad ways to say it, and you're right that being clear on your boundaries enables the good way - can be as simple as "I am sorry, I am just not comfortable with that."