I think my new neighbors are stealing my WiFi?

[!!!]

These new neighbors live right across the street from me and just recently found something out.

Few weeks ago I notice this teenager (which is my new neighbor) stand in the middle of the street or sitting on the side walk next to my house with his phone out. I would always wonder why the hell he always do that. Then it has finally hit me that he may be using my WiFi.

For that, I unplugged the power to the internet box and I looked out my window to see what he does when my internet is "off". About a minute later he goes back into his house.

I tried this for a few days when I would see him stand outside near my home and I am now convinced that my neighbor is stealing my WiFi. I say this because he would always go back into his house when I unplugged the internet.

What should I do?