All you had to do was log into your router and password it to make it secure. When I first went aDSL, I didn't have a password on mine because I thought I had to put the password in every time I wanted to use it. Turns out that wasn't the case. 

Given that half the neighbourhood has been accessing your unsecured router, first thing I'd be doing is changing all your passwords, especially net banking (which you should do regularly anyway). It may not just have been the neighbours using your connection. You'd be surprised how far away someone can be and use your connection with ease. My brother used to own an ISP and his best friend who lived 2 doors up across the road used to use his connection with no problems at all.