Need basic content filtering for open access wireless in business location

We have set up a vlan to provide free wifi to our business customers, but we want to block access to porn and other types of sites that are unacceptable on a business environment.  I don't want to have to install a separate firewall because of the cost.  Can something like opendns (or other software) do this for me?  I'm not clear whether or not this has to be installed on each client or if it is set up to block all users who access the wifi.  We obviously can't install on the clients because we never know who will be visiting the office.