Spam bots make accounts on the site all the time, and there's very little we can do to try and find them out before they make their presence known.
Also, there have been account that spam as soon as they're created, others than do so after a week and others that do it after a longer period of time.
When we find one account that spams, we go looking for other accounts made around the same time and see if they look to be spam accounts waiting to be used. If they look funny, they're flushed out. If we see IPs that have multiple spam accounts, we put them into the IP ban list.
We're working as best as we can to get get of spam as we find it and before it shows up, though we're a big site with lots of users and hard to find them all.