How do I completely disable the Guest account on Leopard?

One change that I believe appeared in Leopard (Mac OS X 10.5, that is) is that allowing file sharing from Guest users is now different to enabling remote login to the Guest account. Sounds like they should be the same, but as you'll see in a sec, there's a big difference between them.

Just as importantly, the default configuration for a Mac is now that the Guest account is enabled for file sharing but disabled for account login. I don't know that this is the best possible configuration, actually and would much rather have systems be secure by default rather than relying on the savvy of the user (and their attention and memory).

Fortunately it's really quite easy to fix this small security hole.

Simply go to Apple --> System Preferences and choose Accounts:

Click on that and you'll see a list of accounts on your computer down the left side. One of them will be Guest Account. Click that and you'll see what's going on:

I've highlighted the important checkbox you need to change in the image above: "Allow guests to connect to shared folders".

Uncheck that.

Now the listing for the Guest account on the left should no longer say "Sharing Only" but instead look like this:

That's what you want: "Disabled".

Now you should be safe.