Inside your s2Member PayPal Pro-Form shortcode, you will see a shortcode attribute that looks like this: accept="paypal,visa,mastercard,amex,discover,maestro,solo"

If you only want to accept PayPal, simply change that to accept="paypal".

If you only want to accept PayPal, VISA, and MasterCard (if you're using PayPal Pro, for example), then you can change that attribute to accept="paypal,visa,mastercard"