I am using DNN 4.6.2 and I turned on Active Directory Authorization for a portal but now find that I want to disable it.  The "Enabled" check box on the portal authentication settings does not update.  What changes to the DB have to be made to turn off AD authentication.

Yes that's a bug that'll be fixed in the next release. In the meantime you can disable it completely by logging in as Host and going to Host->Host Settings and expanding Authentication Settings and unchecking Enabled there followed but clicking on Update Settings. This will turn it off for all portals. Otherwise the settings should be under ModuleSettings but I'm not near a db that I can check.

I had the same issue (DNN 4.6.2).  I tried to fix the issue by:
"comment out the <add name="Authentication" type="DotNetNuke.Authentication.ActiveDirectory.HttpModules.AuthenticationModule, DotNetNuke.Authentication.ActiveDirectory" /> section from your web.config file."  

and:
"Also, go into the properties of the WindowsSignin.aspx and uncheck Integrate with Active Directory."

But that did not resolve the issue.

Also note that the solution of:
"logging in as Host and going to Host->Host Settings and expanding Authentication Settings and unchecking Enabled there followed but clicking on Update Settings."
works for ALL portals, in my case I only have one so it works.  However, if I used this method and then later added another portal (site B) and wanted AD Authentication on 'site B' only, then AD Authentication will come back into play on 'site A', this might not be desired.  Something to keep in mind

To remove AD from the portal I went to the database table "ModuleSettings" and changed 'AD_WindowsAuthentication' from True to False.  I also had to remove the information from 'AD_AuthenticationPassword; AD_EmailDomain; AD_RootDomain; AD_UserName' as I did not want the cruft and it would not clear out using the Admin > Authentication within DNN.

Thanks Mike for the tip.

Hope this helps.