Ah, well, I'm one of the smug weirder-than-thou SOBs who uses Opera, for which it is just in the standard preferences menu. I suspect for Firefox you need an add-on for that (like for most awesome things Opera does natively); NoScript, I think, will let you make a by-site javascript blacklist by turning on trust-globally and then marking individual sites as "untrusted": here's their instructions, but I've never used noscript myself.

Safari I haven't used for a very, very long time, sorry.