Site configuration settings to stop ad postings

Hi,

I have a site https://www.advertisingsubmit.com, this is a classified ads site and it continues to post ads and register users like crazy! I have no idea how to stop it or what I should do within wp-admin to stop this. This site was created by someone for me but they are now un-reachable, I have been contacted by my host saying its taking up too much CPU space. I dont need all these ads or users I think its a lot of junk. Question is how do I stop future ad postings and registrations? Surely there must be something I can do inside WP-admin?

Thanks so much!!