I try to delete a name servers set under
Top > Service Director > Domain Manager > Name Servers > Name Servers Sets > Name Servers Set name > Delete
but get the message
The Name servers set is used by hosting plan(s) # and cannot be deleted.
The Name Servers Set in question was used in a hosting plan that had already been deleted.
There is the software issue PBAS-22030 that will be fixed in future PBAS releases.
Until that, to correct the issue please contact Parallels support for a free ticket.