Once you have a way to backup data per user, just encrypt them with random key and once the account is deleted, delete the random key. We have done it this way,

Once you have a way to backup data per user, just encrypt them with random key and once the account is deleted, delete the random key. We have done it this way, with having multiple live copies of backup key table on multiple locations and beeing backed up daily purging previous backup. The hard thing was to group the data in a way where we can encrypt them with users random key. I hope I was helpful :)