You can use the show hide validation responses to have the form elements become enabled if they choose no. To start this out you would need to set a required validation on the select list. One of the options in this list should have a value but not the other one.
You then apply the enable/disable response validation after this required. You should give this a try with a single form element first to get a feel for how it works and needs to be configured.
If you set the 'yes' option to have a value in the select list and do not specify a value for the no option then you would specify the triggering form element as this select list and if trigger is not valid disable the form element. Please give this a try and if you have any questions about any part of it post back and include a link to what you have up.