They have the right to refuse service to you.
First, the firm is a debt relief agency according to the U.S. Bankruptcy Code. We help people file for bankruptcy. We also do other stuff and we do it well, but Congress wants me to post this notice. Second, nothing on this site is legal advice. You are not my client unless you enter into a written agreement signed by you and me.
Can they kick you out? Almost definitely. Are you forced to accept THIS settlement re: the lemon power system? Almost definitely not.
The claims and rights of Costco against you are independent of any claim you have against Costco. You don't have to accept their settlement offer (but then you better move forward promptly and sue them) and they don't have to accept member under their terms of membership.
The above is general legal and business analysis. It is not "legal advice" but analysis, and different lawyers may analyse this matter differently, especially if there are additional facts not reflected in the question. I am not your attorney until retained by a written retainer agreement signed by both of us. I am only licensed in California. See also avvo.com terms and conditions item 9, incorporated as if it was reprinted here.
They do not have to keep you as a customer.
As for the settlement and your options, stop dealing with them yourself and get with an attorney before you take any other action. You can contact one of the counsel here from your state or other counsel of your choice.