First thing I would say is if someone has hacked into your WiFi connection, you might want to take some time to address how you can improve the security of your WiFi network. I don’t know your level of experience, so please forgive me if you’re already informed on some of this. I’d recommend looking it up yourself in depth, but SOME of the basic things you could do to secure your home network would be to use WPA2 with AES encryption (never use WEP). This isn’t hack proof, but its one of the better options available for regular home WiFi security. This is just one component of securing a WiFi network, though. I would recommend taking it a few steps further by setting up other layers of security, like enabling MAC address filtering, enabling intrusion detection if your router supports it, changing default router admin login credentials, make sure remote admin access is disabled in the router, and beefing up any other kind of security features that your router has available. On that note, it would be worth investing in a quality router if you don’t already have one. I personally use the NetGear Nighthawk AC3200 router, which has tons of security features and is blazing fast, but that’s on the high end for home routers. Make sure your router’s firmware is up to date as well. Also, change your WiFi password/key on a regular basis. The more difficult you make your WiFi network to infiltrate, the less likely you’ll be hacked. Most hackers, or WiFi leachers, are looking for the low hanging fruit. They don’t want to spend a ton of time and effort trying to get into your system to steal your bandwidth or whatever.
The second thing I’ll say is you could change your WiFi key back to what it was previously to allow the machine to re-connect to your network. Then you can use some network tools to try and find out more information about the device.
I’m not sure what OS you’re running, but here are a couple tools that might be a good place to start your investigation:
LAN Spy (find out info about the machine):
Wireshark (packet sniffer - try to find out more about what is being uploaded/downloaded):
I don’t believe the MAC address is that valuable to you, especially since these can be very easily changed/spoofed. I would try to compile as much info as possible on the device and its activity on your network as you can. You might gain some insights after having more information at your disposal.