Since you already gave them the password again, they know you're a bit of a pushover. Many of the replies above haven't figured that out. They also don't realize, don't piss off neighbors when possible.
Solution: just wait a few days so it's not too close to the time of the new password, and simply disconnect your modem from the router. If your device allows it, you can simply run plug in directly to the modem and skip the router; though you lose some of the security it provides. Wait until they mention it, and "yeah, I forgot to pay the bill... I'm not sure if I'm going to even bother getting it reconnected, since I'm the only one in my apartment and hardly ever use it." Wait 2 more days, change the SSID to something that implies it's from somewhere besides your apartment, and stop broadcasting it.
This way, they are able to log in to the router, but will have no Internet access. It will look like the problem exists beyond your apartment.