What should I do about seemingly non-human user registrations?

I have only very recently put a Wordpress site online with BuddyPress+bbPress, have published only a few short posts on it for experiment purpose only and am still far from getting the site to work properly. But I have already been receiving several new user registration notices on daily basis -- the number has reached 112 registered users and still growing (strangely, most of them with @hotmail.com email addresses) and doing no activity on my site even though 92 of them have been given the 'participant' role. And I am also puzzled by the fact that 20 of them are only 'subscribers' when my forum setup automatically gives all registered users the 'participant' role. So, I am wondering what I should do about these seemingly non-human user registrations. Should I email each of them a polite request to register all over again when the site is officially launched? What plugin(s) would you recommend that will help me have only real human beings registering on my site?