Public network security

Hi all:

I am curious how places like McDonalds, Starbucks, etc. with their free WiFi secure their users on the network.  How do they prevent them from accessing/seeing other pc's?  Or, a public library for that matter?

My impression from some research I have done is content filter.  I thought that blocked/filtered out certain keywords or IP addresses.  Can it be set up to block access to other machines on a given network as well?  

I would be grateful for any answers provided.

Thanks.