Is there any way to send a popup message from a Linux systen to a "random" laptop/tablet/mobile linked to my Wireless network ? For example, if I let my WLAN open and I see a not recognized computer connected to it, is there anyway to send to that device a message ?

On the other hand, if I am connected to someone else open network and they may or may not be aware that their network is open, can I send them a message warning that I am accessing their network?

Thanks.

PS: I have no practical application for this is just curiosity.