There are two ways to say 'no'. Either say you can't, or say that you won't.
Here's both:
Stuart, Big Bang TheorySheldon, I’m working. I can’t take you to the dentist. Also, and I can’t stress this enough, I don’t want to take you to the dentist.
Stuart, Big Bang TheorySheldon, I’m working. I can’t take you to the dentist. Also, and I can’t stress this enough, I don’t want to take you to the dentist.