I got it too...
The only thing I can think of is the spammer is using some kind of bot or just manually to scanning the posts/sigs and memberlists of random forums to harvest email addy's.
We did have a hit from someone who found us by using the key phrase in Google "powered by phpbb". This would have brought up every phpbb forum on the web which, unfortunately, included ours.
We do check every new members account to make sure they're legit before we approve them so that keeps the rubbish off the board at least.
Not really any fool proof way round it m'afraid
Best just to ignore them and take some comfort in knowing that we're unlikely to be the only forum community online who this person found
We could set the board to private so only registered members can access it but I'm not sure how many new visitors would just surf away rather than take the time to register a new account.