Well I found a manual, and I've never seen a router setup the ways yours appears to be.. Not that its a bad thing, it has some interesting features, they just lack proper implimentation.. Like why allow profiles for access when you can only control how much net access, and not block it completely, etc..
In any case.. I think you are going to have the best chance, by setting your firewall mode to Custom (it says this is required) and using IP based filters.. Since I have never used the router I can guarantee the necesarry options to block net access will be there..
The manual outlines everything pretty easily too, so if you actually have a real copy handy you can consult that, or I'll go back and find you a link..
There's not too much I can recommend in this case though.. Other than simply getting another cheap router for the purpose you have in mind - and NOT connecting it to a cable/dsl modem, etc.. You'll get DHCP and automatic network stuff, just no internet obviously..