There are a number of helpful AirPort guides. Two that I would recommend are:
To directly answer your question about preventing access to the base station administration from network guests, you can set a Base Station password. Only users, that know this password, will be able to administer the base station.
You can set this using the AirPort Utility: AirPort Utility > Select the Time Capsule > Manual Setup > AirPort > Time Capsule tab > Time Capsule Password