
If you’re having a problem with WordPress trackback spam, then there are few different ways to go about combating it.
Turn Off Trackbacks
The first method is pretty obvious – just turn trackbacks off.
To do this, go to Settings > Discussions > uncheck “Allow link notifications from other blogs (pingbacks and trackbacks).”

So this is fine, and a sure fix, but what if you want to allow legitimate trackbacks? Well, there are few different plugins you can choose from.
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Simple Trackback Validation – Download
The Simple Trackback Validation with Topsy Blocker plugin specializes in blocking trackback spam. Basically it does it by going through a two-step process.
Step 1
First it checks the trackback sender’s IP address to see if it is the same as the IP address of the server the trackback URL points to. As many people who send trackback spam use programs that don’t actually originate from their own website, this first step will catch many if not most trackback spam comments.
Step 2
In this step the plugin checks the website that is supposed to have linked to you to see if they actually did.
Here’s a quick (albeit partial) look at some of the settings

By the way, you will notice the plugin has a “Topsy blocker.” Topsy is a social search engine that seems to get used for trackback spam, so the plugin blocks everything from Topsy.
WP-Hashcash Extended – Download
While not exclusively dedicated to blocking trackback spam like the plugin above, the WP-Hashcash Extended plugin also checks the sender’s IP and also checks their website to make sure an actual link exists to your post there.
Here’ a quick look at the settings for that.

Anti-Spam Bee – Download
Like WP-Hashcash Extended, this plugin is not dedicated to trackback spam, but it does offer an option to check trackbacks and pingbacks.

So there you are. If you’re having issues with trackback spam, try one of the solutions above.
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Thanks to freezelight for the spam image.
I’ve just started receiving trackback spam on my blog and stumbled across your post. I’ve been using WP-Hashcash Extended for about a week and have noticed a huge drop in spam. Thank you!
Glad it worked!