I and many others have found the plugins don't work but you can create your own blocklist using a browser's native functions.
Eg. in Chrome, open Settings and click on Manage Search Engines.
Scroll down to the bottom and add a new search engine. You'll have to add a duplicate google (call it whatever you like) and make that your default search engine because Chrome won't allow you to modify the search string for its google entry.
In the right most field the base search string would be something like this (I live in oz) where %s = your search terms:
To exclude trash sites from your search, add them after the %s in URL format, eg. excluding 'Junk Site' and 'Spam Factory' would be:
If you use Chrome's omnibox (the URL / search bar at the top) it will automatically append this to your searches, eg.-"Junk Site" -"Spam Factory". If you use google's search box (the one between "google" and the little microphone) your search it won't, i.e. your search will be unfiltered.