Joyce was recounting a story to me about a discussion she had with a client. In it, she shared that she told the client there was something she couldn't do. "Couldn't?" I asked. "Well, I can do it," she replied, "I just don't do that kind of work."
"Can't" means you're not able. Don't use it when what you mean is that you "won't" or "don't" do something. It may seem less confrontational, or more comfortable/nice to say you "can't" do something, but it's most often not the truth. You deserve to say the truth, and your clients deserve to hear it.