Type your search terms and the add:
-myvidster.com
to the end of your search string. No space between the - (minus) and the search term you want to exclude. Google will omit any word or domain that follows the - character.
If you want Google to give you only results from Pornhub or some other website, use 'site:' to limit your search to just one domain. For example, if you wanted to search for Cade Maddox on Pornhub, type
This will return all 'cade maddox' results from Pornhub.
Cheers!