1. Activate Akismet
2. Add this in the .htaccess file.

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RewriteEngine On
RewriteCond %{REQUEST_METHOD} POST
RewriteCond %{REQUEST_URI} .wp-comments-post\.php*
RewriteCond %{HTTP_REFERER} !.*yourblog.com.* [OR]
RewriteCond %{HTTP_USER_AGENT} ^$
RewriteRule (.*) ^http://%{REMOTE_ADDR}/$ [R=301,L]

Replace yourblog.com with your blog address.
It block all comments from direct access to wp-comments-post.php.

3. Add this to the .htaccess file also.
Block all spammer IP’s one by one:

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<Limit GET POST>
order allow,deny
deny from 78.133.218.68
deny from 219.159.67.187
deny from 77.41.123.182
allow from all
</Limit>

Just add more IP to this list.

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