How to prevent someone from using our Wi-Fi?

My brother bought an iPhone from our neighbor. I was setting it up and it asked to connect to a Wi-Fi service and so on. So when I was almost done setting it up, it said that the person before had there account still attached to the iPhone. So my brother went to ask him if he can remove his account and he said that someone owned it before him and he didn't know how to contact him etc. So when I found out, I turned the iPhone off hoping the Wi-Fi won't be connected to our Wi-Fi but it still was. Then the next day my brother returned the iPhone but it was still connected. What can I do to remove the device from using our Wi-Fi? (Sorry it's long and if it doesn't make sense IIm sorry)

Change the password on your router

You have to use the MAC Filter.

Add a password.

If the iPhone is not within 75-100 ft of your Wi-Fi it's not connected to your Wi-Fi. You didn't say what you were looking at in order to see what's logged on, but if it's your router, try powering it off for 15 secs, then turning it back on. Only the Wi-Fi devices in your house will log back on. I have a feeling you were looking at a screen that doesn't get updated very often.

If you're feeling extra paranoid, change your WPA2 passcode.

Unless he is still within range with his "stolen" iPhone he tried to pawn off on you. It is simply an echo left in your machine. Some Wi-Fi software stores all devices that have ever been connected to it and shows them as being still connected. Resetting the software (changing the password periodically is always a good idea anyway) will eliminate it from the memory. That can be done by renewing the screen usually, and always happens when the password is changed. Be advised that the password was entered into the iPhone's memory, so the neighbor (who obviously is NOT a very law abiding citizen if he tried to sell you a stolen iPhone) has your current Wi-Fi password.

Yes, he knew it was a stolen iPhone. The big "excuse" is always " I don't know how to get in touch with the previous owner to have him change it". That tells me (and law enforcement) that he tried to use it himself and when he found it was locked by the previous owner, tried to make some money, or at least get what he paid back. (possibly because he bought it from a different thief as well!) Attempting to sell stolen property is the same under law as actually stealing it, so he IS legally a thief! He probably refunded your brother's money so that you wouldn't turn him in to the police.