How to delete a trusted s/mime certificate for a contact in iOS7 How do I remove a s/mime certificate for a contact in iOS7 mail.app?

I'm using s/mime for two-way encrypted emails with several colleagues. A new colleague tried to install s/mime in Outlook for Windows and he sent me a signed email which I opened in iOS and trusted the certificate. 

Then he changed his mind and stopped using s/mime (trouble with Outlook). iOS still tries to encrypt the email messages I send to him so he sees blank messages. I can find no way to list the certificates I have trusted for my contacts so I can remove this certificate. 

Where do I have to look?
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: