How to block TV from accessing WAN through DD-WRT router

Can anyone tell me how I can block a TV that's on my wifi from accessing anything outside the LAN? I do want the TV to be able to access local PCs however, so I can use its inbuilt apps to play videos from a local DLNA server on my HTPC. But I don't want it to be able to access its firmware server, ad servers, or anything else outside my house. My router is a TP-Link TL-WDR3600 v1 running DD-WRT v3.0-r46788 std.