End their subscription on your dashboard: the ⋯ on their row, then Cancel at end of period or Cancel immediately. Taking the role off in Discord doesn't stop the billing, and flatpass puts it back while they're still paying.
In the member list, press the ⋯ at the end of their row. Each choice asks you to confirm, and says what the member loses and when.
Cancel at end of period. They keep the role until the time they have already paid for runs out, and are not charged again. Their row then says when it ends, and the same menu offers Keep subscription if you change your mind before then.
Cancel immediately. The role comes off now and they are not charged again. You can also refund their last payment in full from the same step; for any other refund, see how do I refund a member?
Because the subscription carries on: taking a role away never touches billing, so they keep being charged for access they no longer have — and a member in that position is the one who disputes the charge with their bank.
So while someone is still paying, flatpass puts the role back and tells you. If the same member’s role is removed again within a day of that, flatpass stops putting it back and waits for you: their row says access removed, still paying, and you either give the role back or end their subscription there.
Ask in the flatpass Discord, or email support@flatpass.io. Both are read by the people who build it.