There was an issue with the website causing the deletion of data to not sync back to devices, but we are happy to report that it has been resolved.
Now, if you delete individual waypoints, tracks, routes, or maps, or if you mass-select and delete your data, it will no longer appear on your device once synced.
You can also toggle your data not to sync to your device rather than deleting it completely.
Check out our Knowledge Base article about using GaiaGPS.com to learn how to delete and toggle not to sync from your browser.