How the enforcement works will depend on whether you wish to have your own Radius server and webserver to host the redirect url(they can be on the same machine) or whether you wish to use something like Welcome to the WorldSpot hotspot management system .
You will find that the basic portal is built into some wireless routers with the use of 3rd party firmware such as DD-WRT Main Page - DD-WRT Wiki
Chillispot - DD-WRT Wiki