kma9090 - 9:04 pm on Jul 11, 2013 (gmt 0)
I too have had a similar problem.
I redirected a forum that I owned to my main site as I no longer needed it and because it got a few hits a day I thought it would be fine.
The forum had got a LOT of spam (like a million spam posts) over a few years (the forum was abandoned). I deleted the DB and thought that was that.
Unfortunately on 6 June my main site got hit and I found out I had 900,000 back links pointing to the forum. As I has redirected the forum to my main domain this is why I had got hit.
I disavowed and redirected the forum to another domain with a click to access link.