ALERT ALERT ALERT ALERT ALERTThis system is interdicted by Imperial Law and enforced by the Imperial Navy.
At no time are you to approach the mainworld or scan the mainworld. Non-compliance will be met with extreme sanction.
All vessels within 100 planetary diameters of the mainworld must broadcast their credentials and leave at once, or be boarded and inspected. Non-compliance will be met with extreme sanction.
All unauthorized vessels within 10 planetary diameters of the mainworld may be shot down without warning.
If you require assistance, shut down all engines and transmit SIGNAL GK across all spectra. Be advised that falsely reporting SIGNAL GK is a crime and may result in fines, seizure of ship and/or cargo, and up to 10 years in prison.
All ships that wish to refuel are instructed to follow navigational beacons to gas giants. Staying longer than is necessary to refuel will result in boarding and inspection. Non-compliance will be met with extreme sanction.
ALERT ALERT ALERT ALERT ALERTThis system is interdicted by Imperial Law and enforced by the Imperial Navy...
A text-to-speech conversion of the above message may be found here, if you wish to play it aloud for your game.
In my universe, messages such as these are played using synthesized voices just on the other side of the uncanny valley; human enough to be recognizable, but artificial enough to remove any doubt that there might be someone behind the message capable of empathy.
An artificial voice also makes it easier to tailor messages as necessary (replace mainworld with forbidden location of choice, replace Imperial Navy with Scout Service, etc) with just a few button presses.
Naturally, any system interdicted by the Scout Service will have fewer "Comply or we will use you for target practice" threats, but you get the idea.
A similar message for Amber Zones, transmitted by navigational beacons, could be used to inform travellers of the nature and duration of the hazard. As this message would be made by the Traveller's Aid Society, the tone would be quite different... perhaps a pleasant female voice rather than the harsh male used for Red Zones.