How to limit disk space usage for a particular customer?
This functionality is not implemented in Plesk. Suggest a new feature at plesk.uservoice.com.
The top-ranked suggestions are likely to be included in the next versions of Plesk.
As a workaround, consider limit the disk usage on service plan or on subscription basis:
Warning: the workaround below is for Web Pro and Web Host editions only!
Via Service Plan:
Set the required disk space at Service Plans > Service_Plan_Name > Resources tab > Disk Space: