You can turn off trackbacks completely from your WordPress settings but remember this will completely remove any future trackbacks from all of your single post pages and also it doesn’t provide you any solution for past trackbacks. It only works for future. So instead you may try using following Simple Trackback Validation with Topsy Blocker Plugin.
Simple Trackback Validation with Topsy Blocker, is a WordPress plugin which validates all incoming trackbacks and stops all spams by blocking their requests. Whenever you site receives any trackback, it automatically checks it for for topsy.com trackbacks and marks this as spam. It checks if the IP address of the trackback sender is equal to the IP address of the webserver the trackback URL is referring to, which reveals almost all trackbacks.. This plugin retrieves web page located at URL included in trackback. If the page doesn’t shows a link to your blog, the trackback is considered to be spam. Hence it saves you from trackback spams.
So if this is the plugin you need then install, activate and configure it from admin interface. Here is one more similar plugin for stopping spams.