FormBuilder - Stop Email Subject Field modifying subject

[daymobrew]

Using CMSMS 1.9.4.2 with FormBuilder 0.7.

I have a form with two fields that send emails - "Email to User-Supplied Email Address" and "Email Results Based on Pulldown"

As the pulldown text is not a suitable subject for the emails I use an "Email 'Subject' Field" to change the subject.

I only want to change the subject of the Pulldown email and have enabled the "Allow 'Email 'Subject' Fields' to override the subject specified in the pulldown." checkbox.

There is no such checkbox in the "Email to User-Supplied Email Address" field. Can I prevent the subject of those emails being changed?