I both do and don't. I struggle with these things a lot; though at the office my management's support has given me more confidence in saying no... how to do that outside a supportive context is a lot harder.

However. I do know of a book roughly on the subject; I had gotten the version oriented towards the workplace near the end of my tenure at a bad place, and then never had time to read it. This is the original, more personally-oriented book: https://www.goodreads.com/book/show/133203.The_Gentle_Art_of_Verbal_Self_Defense