ASP.NET Membership unconfirmed user accounts - asp.net-mvc

Question

I'm writing an ASP.NET MVC application, which uses the Membership database to store user registrations. I use email addresses as usernames. When a user registers in my app, I send out an email-confirmation to the address they have used during registration. i.e. I send out an email with a link, which the user is supposed to click, to verify that the address belongs to him.
Until that link is clicked, the account remains 'Unconfirmed' (i.e. EmailConfirmed column equals False). Which means, the account is created, just not active.
How do I deal with a hacker who brute-force creates accounts? I see two big problems here:
Ever-increasing size of the Membership database. A single user, from
a single computer is not a threat, but what if he has 'zombie'
computers?
If User1 creates account with User2#example.com email and
User2 ignores the activation email, the account will essentially
remain locked (unconfirmed), but existing. If User2 decides later to
actually create an account, they can't use their email to register
(account already exists) and they can't Reset Password either -
because even if they reset the password, that does not necessarily
Activate the account.
As for 2) I see a couple of options:
Set expiration date on unconfirmed accounts - i.e. allow the username/email to be claimed again, if the email is not confirmed with 24hrs
Modify my Reset Password method to also activate the account, if it has not been activated. Is that a good idea? I mean, the person would receive an email for that, which is essentially a confirmation, if they click the reset password link in it.
Anything else?
What about 1)? How do I protect myself against bulk create of accounts? Aside from limiting 1 account per IP, per day, using code.

Answer 1

One simple way to deal with this kind of problem is crude but effective.
I usually add an additional field to the form that doesn't form part of what I need - but has a legitmate sounding name like 'Company' - and then I hide it from screen view using CSS. Real user's will never see this on screen, but a bot crawling through the HTML will find it.
Then, when the form is submitted, first I check to see if that form field has a value. If it has - I stop the page from executing any further or just return an HTTP Error as in 99.9% of times only a bot would have filled out that field - not a real user.
//anti-bot measure
if (!String.IsNullOrEmpty(Company.Text))
{
HttpContext.Current.Response.StatusCode = 400;
HttpContext.Current.Response.Status = "400 Bad Request";
HttpContext.Current.Response.End();
}
//carry on processing the form...
I've been using this technique on forms for several years and it's been extremely effective!