How do i throw someone of my wifi?

My neighboor was visiting the other day, and he used our wifi while he was here, he got the code and all. Yesterday i started noticing that our wifi was slower than usual, so i went in and checked, and all of our three neighboors are now using our wifi. How do i throw them of our wifi? i can't seem to find a place to do it, on the zyxel page. I can't seem to find a place where you can change the wifi password either. please help 

#Others_Nov_2018