I had to say no to group therapy to my T a long time ago. (In hindsight I wish I had gone, but that's another story.) I just said no, I'm not willing to do that, and then I didn't do it. I think she kept forgetting that she suggested it because it came up a few times. After the first time, I started to calmly tell her that I had already declined, and her refusal to take no for an answer was starting to make me angry. Eventually she got the message and dropped the subject.