You absolutely can negatively impact the neighbor. imagine he is waiting for a long download to finish and is DLing at max speed. you start using his Wi-Fi, he now is not getting maximum download speed b/c u are using some bandwidth, thus delaying his download finishing. so if he is waiting for an important download to finish you are stealing his time. even if there is no physical theft, i think us Jews need to stop worrying whether it is purely assur or not, and whether it is the right thing to do. You make sure your kid is on time for minyan and reprimand him if he is late to teach a lesson, and then sit down to your “technically Halachikly (maybe) OK stolen Wi-Fi? Sheesh!