How do I remove someone from my network

If this is an exercise for a device you are aware of and own and if you can manage your router, you can remove its entry from the routers DHCP table. One thing to be aware of is that some routers will not allow you to remove a device unless its no longer actively on your network.  So you will need to shut the device down, wait a bit and then remove it.

If this is a malicious device and if whomever added it knows your Wifi SSID and password, they can rejoin again unless you change them or, if your router has the capabilities of blocking a device, add the offending device to its blacklist.