Safari cookie managment

Sometimes (on very rare occasions) I'm using Safari 4 to try out a website or two. And everytime I do that I notice that Safari won't forget the cookies from past sessions. The cookie managment is a friggin' joke, I wonder why we're at version 4 but the cookie handling knobs are no better than the ones in IE5. Luckily knows a way to enforce session cookies:

Right-click on ~/Library/Cookies/Cookies.plist, click "Get Info" and lock the file. Strangely enough, chmod'ing the file to read-only (or even to 0000) did not help at all.