Expiration of unvalidated user accounts?

On my site, I get robots registering tons of fake accounts. Fortunately, the bots all give garbage emails, and are unable to validate their accounts. Thus, I get a ton of accounts that serve no purpose. If an account is never validated, then will it expire after a certain amount of time? And if it doesn't expire, then I think that would be a great idea. A 3 month old account that hasn't been validated is never going to be validated. Heck, a week would be enough.

Am I the only one who gets these automated new users? I assume they would be posting spam if I allowed unvalidated accounts to post.