The setting is:
"Enable double e-mail entry required on contact form"
It is located in the E-Mail: section just under the "Optional E-mail Subject List"
If you change a label by adding your own replacement and want to reverse that, just delete the replacement label.
If you have settings that do not take or re-appear the way they were before, it could be this problem
http://www.fastsecurecontactform.com/settings-cannot-be-saved