Why isn't this .htaccess file working?

Question

Recently, my website was hammered with thousands of POST requests from various bots.

Instead of taking the approach of blocking these bots based on their user agent, I've decided to find a way to block all POST requests sent to my website and allow all POST requests from within my website.

I came up with the following:

RewriteEngine on
RewriteCond %{REQUEST_METHOD} POST
RewriteCond %{HTTP_HOST} !.*mydomain.com.*
RewriteRule ^.* - [F,L]

but for some reason it doesn't work. Any ideas?

Thanks!