Problem Definition

There is currently no setting in Service Desk that prevents Agents from adding customers to a project by using the "Add Customers" button from the Project page:

Suggested Solution

There is a setting in JIRA Admin > Applications > JIRA Service Desk > Configuration that prevents Agents from adding new organizations, so it might be helpful to have a similar setting here as well that prevents agents from adding new customers:

Workaround

1. Disable outgoing email from JIRA Admin > Outgoing email.
   However, this setting will impact the entire JIRA instance and no more JIRA notifications will be sent; or
2. Hide the button via CSS added to the announcement banner: 

    Atlassian Support cannot guarantee the provision of any support for the steps described below as customizations are not covered under Atlassian Support Offerings. Please be aware that this material is provided for your information only and that you use it at your own risk.

   <style type="text/css">
   button.aui-button.js-invite-customers {
       display: none;
   }
   </style>