Blogger Site Exclusivity

If a blogger purchases bloggin rights on a sub-domain site, is it possible for another blogger to purchase rights on the same sub-domain, or is the blogger who purchased first given exclusivity?

And, if a blogger should be terminated, will different bloggers have the ability to register on that sub-domain and purchase blogging rights to it?

(Note: I have a limited number of sub-domain names the bloggers will be allowed to chosse from.)