missymoo on "Author issue with a directory site"

Hi,
I'm currently using Geotheme directory and it seems their is a security concern. I have tried to contact the theme about it but they aren't very helpful.
To allow users to register, create listings and update them it has to be set to 'Anyone can register as an author'. The issue is I have someone adding spam posts and fake registrations and I'm talking hundreds a day. This is taking me hours a day to delete. If I don't resolve this soon I will have to take down the site.
Does anyone know of a plugin or solution that could help with this. Happy to pay someone to fix it.