Currently there is an option to turn off Agents from organization management through - Jira Administration >> Applications >> Jira Service Management -> Configuration

The option -

• No, only JIRA administrators can manage organizations
  Does turn off Add Organizations for Project admins but does not turn off Add Customer

Suggestion -

1. Turn off Add Customer for Project administrators as well (lump them in with agents).
2. Or add another option to control Project administrators the same as for Agents on managing/adding customers

Affects Share in customer portal also -

This also affects the share in customer portal, where it will restrict Agent from Sharing with an email that is not in Jira, which stops them from creating a new customer, but the Project administrator is not exempt and can still Share with an email not in Jira.