How did they find my site?
Some people have suggested spammers can find boards by doing a Google search of "Powered by phpBB". Databases with listings of phpboards are distributed.
Edit: For $495 you get a bundle that includes
Software package includes a databases with links to more than 102,000 forums.
Software is able to work with lots of different types of forums and guestbooks: phpBB and PHP-Nuke with any modifications, yaBB, VBulletin, Invision Power Board, IconBoard, UltimateBB, exBB, phorum.org, wiki, different types of bulletin boards and even custom-written code.