I'm going to politely tell a fellow group chat member not to preach to me.
Why do you feel the need to tell them to stop?
I'm just tired of people preaching to me when all my life I have been a good person.
It's completely reasonable for you to be annoyed because someone in the group chat is preaching to you. I think it's a good idea to let them know that you don't like it, so that they stop.
It's the best way to deal with people preaching to me.
I think that you are within your bounds to be annoyed. I would be upset too if people continuously wanted to preach to me.