Saying no can be a good thing.
No I can’t do that because I have prioritized something else.
No I’m not going to provide that type of service because I am focusing on providing another.
No I’m not going to work with you because I don’t like the way you do things.
But when you publish all the things to which you could possibly say no before you are even asked, people become less inclined to ask you anything, even if it’s something to which you might say yes.
It’s really important to have things that you will refuse to do. You just don’t have to publicize them all in advance.