Problem in disabling canonical 301 redirect

I need to disable canonical redirection of category slug. My url structure for a category page looks like

www.sitename.com/categorybase/categoryslug/subcategory/

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Now when i change the categoryslug in the url to any other word, say catslug, it still takes me to the same page. I want this url to return a 404 page.
My .htaccess file has the following:

# BEGIN WordPress 
<IfModule mod_rewrite.c>
    RewriteEngine On
    RewriteBase /
    RewriteRule ^index.php$ - [L]
    RewriteCond %{REQUEST_FILENAME} !-f
    RewriteCond %{REQUEST_FILENAME} !-d
    RewriteRule . /index.php [L]
</IfModule>
# END WordPress

I think that the solution is somewhere else as the same htaccess file is used on a different website and that site shows up a 404 page when i do the same stuff over
there.

Thanks.