Hello, and Welcome to the HP Support Community!
Do you have the "Wireless Direct" feature turned ON in the printer? I'm guessing that's how they accessed the printer.
If you are not using this feature, be sure to turn it off. If you need to use it, you can add a password to block access.
Wireless Direct turns the printer into a wifi hotspot, visible to those living nearby.
Access the Network settings on the printer itself, and disable the function.
WyreNut
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Former Expert in the PalmOS, WebOS, and Android sections of this Community Forum.
I am a Volunteer here, not employed by HP.