An odd one.
So we setup pFsense with captive portal and were doing fine, adding people by MAC with logins and everything was working, then two days ago without any changes being made to the system it stopped requiring their MAC in the captive portal to reach the internet, and simply began allowing all traffic through. We are using 2.1.3-release of pFsense.
Your post is missing some info.
What does this command show you:
ipfw -x zone1 show
and this one:
ipfw -x zone1 table all list
Note 'zone1' should be the name of your zone - as shown on the Service :: Captive portal or Status :: Captive portal
Tell us a little bit more about your setup.
While you're at it, 2.1.3 is old (login to pfSEnse and it will tell you so) => upgrade.