Set a maximum class size then hide the option

I've set up a registration form for convention classes using radio buttons.  We have three options (classes) per time slot.  Some classes are limited in size (30 participants).  How do I hide one option (class) when it's registration has reached 30?

In editing page, with preferences button you can customize your registration form. At preferences there is a form limits tab, at there you can limit the submission number.

To clarify, My form has 3 days, 4 times slots each, 3 options per time slot.  If one of those options per time slot is over 30, I want it to be hidden, but leave the rest open.

I guess the maximum selection feature in the check boxes can be an option,please try this form and  see if this can fit what you are looking for.

http://form.jotformpro.com/form/50135701197955