I don't think you can do this in the way you are looking for. In the section where you set the coupon, it says:
s2Member accepts Coupon Codes on any Pro-Form with Shortcode Attribute: accept_coupons="1". However, you may configure Coupon Codes that only work on specific Post or Page IDs. This can be entered as a single Post/Page ID, or as a comma-delimited list of multiple IDs.
So you will need to put the registration form for the levels that do accept the discount on a different page from those that do not.