Build a Website

My Joomla website version 2.5.6 appears to have a security problem. Under Components / K2 I am getting rafts of users and comments. I have manually removed the thousands of users but cannot get rid of the comments. None of this is showing up on the website, and I think I might have prevented further invasion, but I need somebody who knows what they are doing to go through the site and make sure its security is up to speed.