Well, that's the whole point of a Guest network on a wireless router - it creates a second wireless network for 'untrusted' computers, and is segregated from the main ('trusted') LAN (wired & wifi.)
If you want to be extra-anal about the segregation, you could run two routers in series (modem > Guest > Main); but, it's a PITA to configure that correctly.
Or, if you have an 'older', unused PC laying around, install Smoothwall Express on it. Even a low-spec'ed machine will run it fine; you need simply install three network cards (Red (to modem), Green (main LAN), and Purple (guest LAN)) in it, and connect switches and wireless APs (or, routers configured to run in AP-Only mode) to the Smoothwall box.
In Smoothwall, Purple is well and truly isolated from Green - nothing on Purple can 'see' anything on Green.
Very stable, very secure, and very configurable to suit your exact needs.