How to delete a trusted s/mime certificate for a contact in iOS7
You should be able to view and remove certificates or S/Mime if you go to:


Mail, Contacts and Calender
Go to your mail account
Click advanced 
Uncheck S/Mime or remove the certificate


Please also check out this Official Apple Guide
By clicking the name in the Mail.App you should be able to revoke the certificate.

Check this image: