I think just bluntly, without a word, cutting them off is likely to result in hurt feelings, but telling them that a problem occurred and the solution made the original agreement impossible to continue, is more polite. And less likely to result in an unpleasant scene where the neighbor promises to pay, and is turned down for having not paid (being a deadbeat, is how that might be heard), asks for more time. Because really, what happens if they say 'I've made arrangements, but the installation guy can't get here til the 30th/I can't get off work to be here for the installation til the 30th?' Do you say, 'Sorry, one week is all you get' or do you extend the deadline? And then on the 30th, the installation guy doesn't show...
I don't understand the thinking on the bolded part. The neighbors are the ones who have already broken up with the OP and voided their agreement when they made the choice not to pay. The breakup has already occurred, the OP just hasn't reacted to it yet so I don't see where its even vaguely impolite to just shut off their access.
As the saying goes, it's the person who hits second that gets caught. I know I have dragged my feet on too many occasions because of the fear of creating bad rel@tionships, being hated, or even retaliation, even though it was the other people who started it and the other person's actions that caused it. Think about the other post about the woman who wanted her money back on a trip and proceeded to trash-talk the OP to all her coworkers. Who will end up thinking the OP is a blankity-blank, even though it's the other woman who is being childish, and how will this affect her professional life? You know deep down it was THEM, not you, but you end up being the pariah anyway. I watched this play out with my sister and her neighbors over a situation. All the neighbors side with the moochers and what horrible people the OP and her husband are. It happens. This is why there is "that thinking."
Hopefully it won't turn out this way for the OP, but I understand the hesitation to stand up and do what you need to do.
I'm glad you made the switch, OP.