How to Remove ‘Related Pages’ Block from Facebook Pages

I'm not aware of a way of doing this directly. However, you can use the plugin from Social Fixer.

This enables you to turn off and/or hide several "features" of the Facebook page like this.

It isn't available for all browsers on all operating systems but it covers most combinations.

If all your likers are not showing up on

Settings -> People and Other Pages

an alternate way to handle the flood of content that you don't want might be to use their posts to remove them.

Each reply you get has an 'X' on the RHS. If you click on that it will Hide the comment. Then it will display several options where the reply used to be displayed:-

Unhide   Delete   Report   Ban <name>

You can then click to Ban that person.

This will only work for comments, not likes, but it will clean up any of the active users on your page that you don't want. Those that only like and don't comment are perhaps less of a worry anyway.