Within Gmail, open the Contacts Sidebar. In the sidebar you can search for a specific Contact or open an email from them. You'll want to confirm this is the correct Contact and then at the bottom of the sidebar you can click "Delete Contact Record." From there, a warning will pop up and you can click "Delete Contact" to remove the Contact from Yesware.
In the app site, you can search and select the specific Contact. From there, at the top right of the Contact page you can click "Delete." A warning will come up and you can select "Delete Contact" to remove the Contact from Yesware.