Allow access to internet but not network from wireless?

I am setting up a network that needs to allow visitors to be able to access the internet from our wireless AP, but not be able to access the rest of the network. We also use the wireless to access the network from our own machines and need to maintain that.

Is there an easy way to allow internet, but block network access to visitors?

It is a fairly simple network with a wireless router, switch and 4 computers connected.