The existing -site:w3schools.com works but it's not convenient *and* you have to know in advance what trash sites you want to keep away.
Proposed: A per-hit option to scrub that site from the results, which reruns the search with the -site: string (implicitly) and retains that preference for the duration of the session.
Unanticipated bizarro trash can be easily cleared away;
No site gets permanently blacklisted (this way) -- maybe tomorrow you don't care if about.com keeps coming up;
DDG gets piles of clean feedback on which results are most despised (probably a lot more than from the Feedback button and with no interpretation required).