Haven’t worked on this directly just yet but going to soon. One of the options we are exploring is a scheduled workflow that checks if a user has 0 entitlements on the source, then uses API to make a call to Box to delete the account.
Hard to say exactly, though it does seem to me that with ISC, there’s a hesitance to delete accounts entirely. There’s been an idea open in the ideas portal for a long time now regarding account deletions, and not much movement on it unfortunately.