Being able to disable layout objects would be a great new feature for FileMaker: http://www.filemaker.com/company/contact/feature_request.html
Since you can't there are a variety of options you can try. Just because this is all in one table doesn't mean you can't use a portal. You can use a self join to link a table to itself.
You can also use an invisible tab control to make the field disappear or you can put a copy on one panel that has a "disabled" appearance and which as field entry prevented and a normal looking editable version on the other. A script can then select the correct tab panel.
You can also use conditional formatting to give the field a disabled appearance and then use the OnObjectEnter trigger to perform a script that checks to see if the field should be disabled or enabled. If it should be disabled, the script can use go to field, go to next field or go to object to move the focus back out of the field in order to prevent entry into the field when it is not permissible.
Here's a demo file that illustrates a number of methods for making layout objects appear/disappear that you may find helpful:
Very cool info, Phil. Thanks.
You're so quick!
I was just getting ready to post back and say that I used the OnObjectEnter idea and it works fine.
I'm using two triggers. One is on the "No" radio button. If they click that, then it zeros out any text entered in the answer and moves them to the next record. If they come back to that record and try to land in the answer field, then I have another OnEnter script that says: "ah ah ah, you have to change the answer to 'yes' first", then dump them into the radio button field.
Funny business rules, I know.