You can delete workout logs in the app and on the website. Deleting a log in any location will now delete it everywhere. Deleting a log is permanent and any deleted in error cannot be restored.
Website
Log into your account and find the log you want to delete. Click on the "..." icon to the right, then click "Delete".
Android and iPhone
Open up your "Workout Logs", tap the log you wish to delete, then tap the trashcan icon next to the title at the top of the screen.
You can also delete logs by swiping them (to the right on Android; to the left on iPhone) on the "Workout Logs" screen that lists all your logs. On iPhone, you can also delete them in bulk by tapping the "Edit" button.