Comment Just sandbox them (Score 1) 572
Just create an ad-hoc guest account with limited rights. That way they can't really screw up things. Once the guest has left the premises, remove the account. You don't even have to log out yourself if someone just needs the access for five minutes, just switch users.
A step further: Build a virtual machine with a e.g. your basic Linux distro or Windows XP, create a snapshot of it in it's "fresh" state, and set it up to talk only directly to the Internet without any access to your local network. You can achieve this with Virtualbox at least. Let your guest access the virtual machine. When the guest leaves, just revert it to the snapshot state.