To delete images on the Theta, the Theta must be connected to wifi.
1. With the camera connected, click the photos button on the bottom right of the Theta app.
2. Click the select images button at the top right of the app.
3. Click the "select all" button at the bottom right of the app.
4. Click "Delete" to delete all files.
It's best to do this every couple of weeks. Don't worry about deleting them from the Theta since you will already have a backup on your camera roll.