I am by no means an IT expert, but, we do have a wireless router and did encounter the same issue as you (though with Bell we have unlimited bandwidth so all we lost was speed, not money...)
What I did was log into the router and assign each computer that needed a wireless signal an IP (or maybe a MAC address, not sure anymore) to limit who could connect. I invented the number so it was virtually impossible to hack.
Hope this helps, and hasn't yet been covered (I got lazy and didn't bother to read above). I can say that while the competition's service is the best, their customer service isn't.