Tracking can be toggled off from your activity feed by selecting the message and sliding the toggle to the left. By toggling off tracking, events, link clicks, presentation views and notifications related to your message will no longer appear.
Within a thread, tracking can be disabled by clicking on the Tracking icon at the top right of the thread, and then toggling OFF Tracking.
New individual messages can disable tracking by un-checking the "Track" checkbox that's to the right of the small Yesware icon in the lower left of a new e-mail before you send an e-mail.
You can also turn off global message tracking by going to the Yesware drop down within gmail, then choosing "Preferences", then "Compose Options", and set "Track Opens" to OFF.
For O365, once you've sent with Yesware, you cannot stop tracking an email thread.