If you've already sent a message with tracking enabled, you cannot turn off tracking for that message.
If you're sending a new email and do not want to use email tracking, either do not use the "Send with Yesware" button (keeping in mind if you have the Enterprise plan with the Sync to Salesforce, this means your email will also not sync to Salesforce) or, if you want the email to sync to Salesforce but not be tracked or if you're using the version of Yesware that does not require the Send With Yesware button:
- Click the gear icon in the compose window taskpane
- Select the "Settings" tab
- Toggle "Track Email Opens" to the "Off" position