How can I block the guest Network user from accessing my Local Network?

Guest Network,

when i connect to guest network sometime i can access the local network, how can i block the access to the local network when i connect to a guest network?

i connect to the guest network but still i can access my local network,

i have the same problem in my Time Capsule and my Airport Express?

please advice me, how can i solve the problem ?

Best Regards,

Fawaz Musabeh

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