That's my understanding - have a look at the thread that I linked to from @ktuite - it says "a daily cron job for purging deleted forms ... will do its work." and being a little 'retentive' I asked the very same question:
Not sure if this will help, but here is a basic pyODK script that works for deleting specific attachments, so should be possible to adapt very slightly for the new delete submissions [remove +'/attachments/'+str(filename) from the end]... You will need to set up the credentials for pyODK - which are in the DOCS.
This would give you the opportunity to loop through a series of submissions and/or attachments in standard python methods - you may be far more skilled with pyODK, so please don't be offended by me offering this:
from pyodk.client import Client
client = Client()
projectId = [a number]
xmlFormId = 'your_form'
instanceId = 'uuid:...'
filename = 'e.g. epoch_time.jpg'
response = client.delete('/projects/'+str(projectId)+'/forms/'+str(xmlFormId)+'/submissions/'+str(instanceId)+'/attachments/'+str(filename))
print(str(instanceId)+' / '+str(filename)+' status: '+str(response.status_code))
(the projectId and xmlformId can be found in Central under the link 'analyse with OData' in the form submissions page)
Also, I don't know how to retrieve attachments that are in Trash - so some caution is required...
Good luck with the deletions and disk space recovery.