Internet Security !!

Hello i have recently noticed unauthorized user on my wi-fi but the problem is that the wi-fi is encrypted.Im not sure how hes getting on their and stealing my internet connection i tried to use mac filter to block him but their ways around that since hes still stealing it from me, is their somethnig i can do? I’m just mostly worried about the security of my online transactions and personel information. I also tried to block his IP adresses but no luck.

ps. thank you for the help.