Good question! My plan is essentially to ask them politely to not. :)
There is a soft limit of 5 GB per subdomain. Once a user hits that limit I'll send them a notification about the limit and eventually take some action like asking them to remove data or moving them to a monthly or metered plan.
It's still a bit of an experiment right now, and I may find that I need to adjust that limit. Thanks for asking! I appreciate the feedback.