Stripe's commission for American Express are higher than for other card types, so you might want to prevent people from paying with amex cards. You can do so by following these instructions:
1) Log in as an administrator
2) Click "Config"
3) Click "Payment Gateways"
4) Click "Configuration settings" next to the gateway in question
5) Enter "amex" into the "Disallowed Card Brands" field
6) Click "Save"
This will cause the amex logo to disappear from the payment page that your customers see, and if they try to pay with an amex card, they will get a message stating "We are unable to accept amex card payments. Please use a different card"