This can only be disabled if the coupon codes never make totals go to 0 as payment methods often depend on total amount and if total is 0 they can be disabled or enabled.
If you wish that the payment details remain after applying coupon code, please do the following:
open up app/design/frontend/base/default/template/onestepcheckout/checkout.phtml or the same template on your theme folder and on line 235 and 330 set
var payment_methods = $$('div.payment-methods').first();
var payment_methods = false;