To close down Safari, you need to force quit it. From the Finder menu, select the apple logo, then Force Quit...
You can now close down Safari from here.
To get Safari working again, I think your best option is to delete all cache files. You can do this manually, or use Onyx or Snow Leopard Cache Cleaner. Obviously, to download these you'll need to use a different browser. After you've cleaned the caches, do a reboot.
You might also need to clean out Safari's history, last session and maybe top sites. You can find these files in Library/Safari. Just drag them to a different folder.