Disable LAN on R-Series Routers

I'm attempting to configure some R-Series routers in ATA only mode. I don't want the customer to have GUI access or any sort of routed access to the WAN port. Support said this is not possible, and I should request this in the Idea section.

I picture simply disabling GUI access from the LAN side, and stopping routing between sides. Right now, GUI appears to always be accessible on the LAN side, but routing can be mostly stopped with incorrect settings.