There is a setting in XF2.x that sets a time on when a username can be reused. It's under the setup, options, user registration, at re very bottom of the page. On the site I run, it is set to 180 days. I don't know if that is something the original owner set it to, it if that is the default but basically you can prevent the recycling of usernames if you want to (and may already be doing so without realising?)