Safari asks if you trust shockwave; how to backtrack

Safari asks if you trust Shockwave for individual sites before opening a video: good

Trouble is I allowed it to trust Guardian/Observer web-site thinking it would be OK, but find a popular video (Niagra falls frozen) has been hijacked by someone showing the most hideously noisy advertising and gambling drivel

How can I remove the "trust" for that website?

Is the facility for this type of hijack why Apple warned one of Shockwave not conforming to its standards, or is there even worse?