1stly, check they registration date of that user. Make sure it wasn't already registered (some sit around for a while before posting)
2ndly, have a look at how it registered. If it came via the registration page, it is highly unlikely it would have been a bot that registered. However, if it came via FaceBook registration (or Google/Twitter Auth), then you have a small leak in your system, and it might be worth considering using an antibot mechanism for these registration areas (see
facebookregcaptcha )
(you need to check your server access logs for that IP address)
3rdly, what was the IP address they registered with (and what location was it associated with). I've noticed it's fairly rare that manual spammers use proxies, but bots almost always use proxies. For this reason, I use StopCountrySpam to stop human spammers (since in my case I can block certain countries), and FoolBotHoneyPot for bots. If the user used an IP address that is associated with a country that "spams lots", it's fairly likely they are human spammers.