We're often told in the self help and spiritual genre that we should avoid negative people and negative influences. As with most concepts, this practice is exagerated to avoid us doing our work. We mistakenly believe we are working on boundaries, in reality we create walls.
In the practice of alchemy, we set out to integrate our shadow to complete ourselves. A completely integrated person is an authentic being. When someone says something that pushes a button inside us, most likely they have hit something that rubs on a part of ourselves we don't like. This is not time to run and create walls, but to go inside and find the origination of the hurt.
My go-to affirmation in this process is that when I'm confronted with something that riles me up, I don't consider how to tough myself to take the blow. I imagine what the person said as a particle of matter. If I've reacted internally to the offending statement, I've discovered some work I need to do. My practice is to imagine the offending statement like a neutrino. A neutrino is a particle that is so small and has the characteristics to travel through matter almost completely unimpeded. The person who grows thick skin becomes calus.
This practice doesn't not nullify the need to understand or use boundaries, it helps in the integration of the shadow. Wisdom is understanding the nuances involved of boundaries.