That sounds like a terrible idea and you shouldn’t do that! I don’t even know if it can be done…
But you could remove the monitoring app and hope that the customer doesn’t figure out how to install it again. Then he can’t see that the effective storage is much smaller than what he pays for 
No seriously! Don’t do any of that!
And I sincerely hope that you have a storage concept that is easy to expand, has a certain redundancy and you already know how you are going to expand it, when the customer actually wants to use the storage you have promised him. Backups would be another thing that you have hopefully thought about and that you also have extensively tested, especially if you host data from customers.