Does anyone know how to "unlock" those to be able to delete them?

Unless someone has a hack or "how to" I don't think you can delete those ringers because they are installed as part of the OS and you cannot access them.

Maybe someone knows how to get into the Pearl's OS [kinda like accessing DOS or Windows registry], but I don't think so. Of course, even if one could, I suspect messing with the OS code is fraught with peril.