You just have to be a really diligent mod. I moderated one of the largest Harry Potter boards on yahoo back in the day and we CONSTANTLY had to keep an eye out. Kids would post their phones numbers or addresses, wanting pen pals or just a chat and we'd have to jump and take them down. The kids thought we were mean, but we were trying to keep them safe. 10,000 something people in a board. It was nightmarish sometimes. And Yahoo had a private chat function we had to disable, otherwise creepos could hit up on kids and we'd never know it!
They thought we were awful meanies, but we kept up the Mad Eye Moody credo of Constant Vigilance!