You could write a php script which will be wrapped as cron-job and will access client products using WHMCS API. So, there is a function on WHMCS API which will allow you to get the client's product called getclientsproducts() where we will pass the client ID. It will returns the product which is bound to client. One of the returned values is Disk Usage and Disk Limit. So, we could retrieve it regularly for your client, and do the necessary action once the limit reached.
I could write the script for you and wrap as a cron-job.