You may also view which devices are connected inside the Administration page at 192.168.1.1 in your web browser.
Make sure all your devices have a name and you will be able to determine fairly quickly if an odd device is connected because most times it will show the name as "unknown".
You could also deselect the Broadcast SSID from the page so it won't emit a broadcast signal but experienced hackers may get around this as well.
Make sure to change the SSID to something totally different than what is the default along with the password for the WIFI, remember the Administration password is going to remain the same as the default and has to be changed separately so be sure to do that as well.
It's possible the hackers may be generating access codes due to the SSID name remaining the default for most devices and if you change it to something totally different that would mix up the attempt for them for a while.
Other than that, change your password ever so often and pay attention now and again to what devices are connected.