To disable the Send to Salesforce for an individual email, click the "Salesforce" cloud button at the bottom of the compose window and toggle "Send to Salesforce" to the off position.
Please note, if your Yesware administrator has enabled a Global Policy that forces the automatic sync to Salesforce, you will not be able to toggle the Send to Salesforce to the "off" position.
You can also ask Yesware to log out of Salesforce permanently by clicking here and selecting "Deauthorize."