Security

Have a roommate who somehow has access to my WiFi and xfinity account l have changed passwords but he still gets access lm not to knowledgeable when it comes to all of this but lm trying to learn he on the other is very tech savvy and knows how to get over on me l think he really gets a kick out of all of this and doesn't have to pay a penny so if anyone can enlighten me l certainly would appreciate it.   Thanks Diane