Without knowing more of what you are doing you could...
1. Set a script trigger on enter that prevented re-entry to the field if the field is not empty.
2. Use the same field twice with a different one item radio for each then use a trigger to prevent entry.
3. repeat 2 and use conditional formatting to hide the copy of the field with no data in it
I am working on Order form of Bakary in which I have to put two type of cake .
1. Theme cake and
2. Custom Cake.
when user select Custom cake ( by radio button), order form show the fields for customize cake(such as color, decoration , flavour etc.)
but when user select Theme cake( already available in bakery) , the customization fields have to hidden or disable .
I am unable to fully understand what you told in your reply, Please reply in brief.
Thanks for your response
If you placed your options - color, decoration - etc. into a separate data table you could use the Custom and Theme values in a key field in a relationship to the options where only "custom" would return valid data in a portal.