In my opinion, the most polite way to refuse is to pretend to be poor. When you pretend to be poor, you may be returned by him, so this is also skillful. Money is the most essential thing, and the coach sells it to you to make money. At first, you can euphemistically say that you like his teaching method, and then say that you have no money to open a card, and that your salary is low and you really have no money to open it. Note, this is what they will calculate with you. How much money will you save when you open a card a year? Don't be fooled, stick to your original decision and say that you don't come often. Maybe he will say, if you open the card, you must stick to it. The most important thing is coming. You said you weren't a local, but you just love fitness, so you came to the gym and left in a few days. Of course, if I see you again, he won't haunt you, and your attitude is so firm. And the fitness instructor is just a coach, not a friend, so don't be embarrassed.
The second method, which is shameless, is suitable for people who are more active. When the coach asks you to apply for a card, you just shirk it and never do it. You just have to pester him to teach you. No matter what he says, you just don't do it, preconceived. In the long run, he will be afraid of you. Whenever he sees you in the future, he will hide immediately. You don't have to hide. Of course, this method also needs some thick skin. If introverts don't recommend this method, because the coaches who will pester you to apply for a card are usually very powerful, you may be at a loss.
Finally, my suggestion is that whether to buy a class or not depends on your own initial intention. Don't buy just because others have been pestering you and then feel embarrassed. In the end, it's useless to yourself. Our money only buys what we like, only what is useful to us, and does not pay for embarrassment.