Trying to hide the contact form on a particular page

If I understand correctly, you’re trying to turn off the ability for users to add new comments on a single blog post. Is that right?

If so, instead of using CSS, I’d recommend using the built-in WordPress feature to turn off commenting for a single blog post. You’ll find it within the “Discussion” section when editing the post. The option is called “Allow Comments”. Uncheck that option, save the post, and you should be all set. Also, don’t worry, you’re existing comments should still appear.

Here’s a screenshot of where the setting is in case you need it: