If you suspect that your neighbor stole your Wi Fi I would strongly suggest changing the encryption key (sometimes called the pre-shared key) of your broadband router. Your internet service provider (the company you pay your monthly broadband bill to) would be the best people to contact if you suspect your Wi fi is being used by someone else. They may also suggest strengthening the type of wireless encryption e.g. using WPA instead of WEP. If possible I would recommend WPA 2.”]