There's nothing you can do other than move away from unlimited storage. The model, as the OP states, works well initially, however over time, "power users" emerge to want to make use of unlimited. The only choice is to remove unlimited storage as a fixed subscription fee. You may have metered billing (unlimited storage but you are charged per GB) or to do away with unlimited storage all together.
Keep in mind, that change is great for the business and 99.9% of the users. The business saves money on resources, and those resources are re-invested into the other 99.9% of users.
Keep in mind, that change is great for the business and 99.9% of the users. The business saves money on resources, and those resources are re-invested into the other 99.9% of users.
Edit: grammatical error