I have same problem. Changing password would be easy, but I want to find the guy and either walk over and slap him/her up side the head. Or use their information to hack their system and steal allllllll their stuff.
Everyone says that. "send virtual bomb" but no one tells me how or volunteers to do it. I have his ipaddress if that is of use.
And, yes I did change my wifi password so he/she/it is no longer on.