With the August release of updates for Garmin Connect, we chose to include many of the most requested features all the way back from the MotionBased requests. You've no doubt seen the Manual Activity and Calendar features. A smaller feature you may have missed is the More Actions feature. Currently, this allows you to bulk delete activities that you don't want in your Activities listing. This makes deleting unneeded data and "dummy" activities much more manageable.
You can find the More Actions button on the Activities page, right alongside the Quick Edit and Manual Activity buttons. We'll add other options to More Actions with future releases.
To get this feature working for you, simply check the boxes next to the activities to be deleted, then click More Actions and select Delete.