What is JotForm?
JotForm is a free online form builder which helps you create online forms without writing a single line of code. No sign-up required.
At JotForm, we want to make sure that you’re getting the online form builder help that you need. Our friendly customer support team is available 24/7.
We believe that if one user has a question, there could be more users who may have the same question. This is why many of our support forum threads are public and available to be searched and viewed. If you’d like help immediately, feel free to search for a similar question, or submit your question or concern.
If I understood you correctly, you want to make some of dropdown field options to be inactive (disabled).
Only way to achieve this is to get full source code of your form and embed your form to webpage. Than you would be able to disable some options.
Inside of this guide you can find how to get full source code of your form:
Here is an example: http://www.w3schools.com/tags/att_option_disabled.asp
Other way is to delete dropdown field options.
Hope this will help. Let us know if you need further assistance.