There can be hardware failures as well as software failures to cause this. Remember that you claim "it stopped working" and not a reduced or slow functionality.
If it's something different then I would not know that. Troubleshooting such can be expensive so why not just ban the interfering machine from the WiFi? Make them get wired if they want access.
Okay so I have an HP Pavilion Elite HPE desktop with the built in wifi and all that jazz. So the built in wifi worked fine for quite some time, however it stopped working, so I got a wireless adapter (the one you plug into wall and hardwire a connection to the PC.) The adapter seemed to work fine, however my wifi would always drop off, the hard wired connection on another PC would be fine, but wireless would be interrupted (for Xbox, iPhones, Nooks, everything.) Called my wireless company, they replaced router and the problem continued. They replaced modem, the problem continued. They were up on ladders checking wires doing everything possible and the problem remained. They contacted their supervisors and all, couldn't seem to figure it out. I even got a USB wifi adapter which cost me a decent amount, still the problem remained. So I simply dealt with the problem of getting bumped, quite frequently, and stopped online gaming. Now I move into an apartment (previously occupied by my brother and still on the same internet connection that he had working fine for 3 years) and the problem begins again. Is there something wrong with my PC that is forcing it to interfere with the wifi?