Open a new tab in your browser and go to https://stripe.com and sign into your account. After you sign in, click on "Customers" on the left side menu. Find the customer you want to refund and left click on their email. You'll then be able to see more details (such as the card used, payment amount, etc).
Now to issue a refund, scroll down to the Payments section, click the three dots on the right, then click "Refund payment":
You can then choose to do a full or partial refund.
If you would also like to cancel the member's recurring or multiple payment plan simply go into your MemberSpace account and click on Members in the top navigation, find them and click "View member details", then scroll to the bottom of the page under "Current Member Plans" and click the grey "Cancel Plan" link.