This document provides the concept of deleting group or specific customers from a single project or even multiple JSM project
Before you begin, please see this document on How to bulk remove customers
The above will allow you to create a CSV file that can help bulk remove the customers from the JSM project you want.
Also note that removing the customer from this project does not delete the customer entirely from your JSM application. This only remove the customer from that specific project or projects.
To completely delete off the customer user, please see this document on