how do I prevent my registrations of domain mapped independent sites

Alright -- I have a series of domain mapped city directories -- and though I want all the registration information to be in the main DB---

i do not want them taken to the main site to register -- I need them to register right on that particular site -- since there is a process they must go through -- to register and then submit a listing ----

or is there a way to immediately send them back to the site they came from -- once registered -