billing error: can select a billing period where prices are set to 0,-, or question: howto

feature probably - to me its an error or at least something that needs a different way of handling.

when you setup a hostingplan you setup prices for month, 4 month, 6 months and 12 months and maybe a different period all together. Then you select the default period.

we dont want to accept "monthly payments". Its to much work. So... how do we do this? Not by setting it to 0,- ! if we do then the user, if he is buying himself, can actually just pick a billing period not listed in the list of possible billingperiods and prices and he will get the product for free.

so error or just a big limitation in the system - meaning HOWTO setup that we only accept maybe 6 and 12 month periods for a hostingplan.