Having the same problem, I want to share my solution using Vanilla Javascript, so no need to use jQuery.

In Contact Form 7 having a select field like this:

[select* my-select-field class:form-control first_as_label "Please Select Option" "Option 1" "Option 2" "Option 3 is not selectable"]

Assumng the CF7 form has class .wpcf7-form

<script type="text/javascript"> document.addEventListener('DOMContentLoaded', function() { var selectElement = document.querySelector('.wpcf7-form select[name="my-select-field"]'); var lastOption = selectElement.querySelector('option[value="Option 3 is not selectable"]'); lastOption.disabled = true; }); </script>

Paste javascript code at the bottom of your contact form 7 after submit button. Enjoy.