You can delete a workout in PowerCenter as well as the Stryd app. After you delete the workout, you may need to fetch the workouts again on your watch to remove the session from the watch.
In PowerCenter, you can select the 3 dots top right of the card in the calendar after which you will be able to delete the workout:
You can achieve the same in the Stryd app by long-pressing the card within the app (same for iOS and Android:
If you have additional questions, contact firstname.lastname@example.org.