Hi noboss, just my two cents worth here. I think that by opening up your Wordpress to register any contributor, you just made yourself vulnerable. It's a full time job (almost) to double check the email addresses.

The anti-spam feature of Wordpress is applicable only to comments, as far as I know. That feature is rendered useless once someone is considered an official contributor.

If I were you, I would stop accepting contributors automatically. Instead, I would invite people to email their contributions instead and I'd be the one to post them and introduce the authors as guests.

I would still invite contributors but on a more prudent manner. I would start with people I know and limit the number of contributors to just five.