How can I remove a RSS feed that a WordPress plugin adds to `wp_head()`?

Without the specfic plugin name, I can only be fairly vague, but there are two things you can do:

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To add additional code to unhook <link /> tag (which is what I presume it’s adding) you would use something like:

remove_action('wp_head', 'name_of_function')

You would, however, need to know the name of the function you wished to remove, which makes simply removing the hook easier.

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To add the <link> tag to your wp_head the plugin will probably use code similar to this:

add_action('wp_head', 'name_of_function')

Obviously it is much easier to find if you already know the name of the function which you are trying to remove, but a search for add_action('wp_head' or add_action("wp_head" should find the line specified, which you can then remove to stop the insertion.