Spambot invasion...

[Outlaw Mantis]

I'm still working on my reboot of a forum before I launch it, but I did open the floodgates and forgot to close when I went out for the day. When I got back recently? FIVE THOUSAND spambots have registered and some have posted. I hadn't thought to install an extra layer of spam protection yet, but I get the impression there is no spam protection on vBulletin by default. I just can't understand how it can happen in such massive numbers. I hate to bring up MyBB again since it's probably the last thing you want to hear, but back when my board was active and on MyBB, I got a few spambots a week. After having those for a while, it's then I decided to add CAPTCHA protection. My point is, what could have caused this? I used to own an active board. Now with a dormant vBulletin board which I accidentally left open for a while, this happens?

I assume I can delete all users in a date range? I think I'll be able to remove the posts manually, but I'm still worried about the future even with spam plugins. I'm confused. I've often left it open for hours at a time when testing features with no registrations.

Can anybody shed some light?