Dear Robert,
Greetings of the day
I would suggest to go to Service Center-> Applications & delete applications which created in training.
This will also delete any modules which are inside those applications.
You need to check only those applications which are created / published in the last week.
Additionally, to clearup environment size & junk data from backend, you can use dbcleaner forge component.
I know this is not the best approach but as its confirmed by OutSystems support that there can not be reset, I would use this option to cleanup the environment.
Let us know if this helps.
Regards,
Palak Patel