For individual sites, I can find no dialog so the above suggestion is probably best but, you can clear all saved passwords by:

1) Open a web page(any it doesn't matter - you just can't do it from the initial bookmarks screen)
2) Click the title bar where he little arrow is.
3) Select "Options"
4) Select "Clear private data"
5) De-select everything and then select "Saved passwords"
6) Click "Clear"
7) Done