Southwest Gas Reports Gas Service No Longer Active But Still Charges Las Vegas, Nevada

I checked my online account after moving out of a place, and verified that gas service was no longer active.  On the main account screen it stated "The gas service for this account is no longer active. Your final amount due is displayed below." with $0 as the amount due.  However, 5 months later they send me a bill for about $50.

I log into my account online and sure enough, it shows the same $50 amount now.  I call them and they basically tell me I'm responsible for paying it because I didn't call them to cancel the service.  I have no proof that I called, nor do I even remember calling.  The only proof I have is my word that the online account said the service was no longer active.

I specifically remember logging in multiple times (over the course of a few months) because they would send me emails reminding me to pay my bill, and that I owed them whatever amount of money.  But I would log in and it would say exactly the same thing: "The gas service for this
account is no longer active. Your final amount due is displayed below." with $0 as the amount due.

I'm writing this because I'm wondering if they're doing this on a large scale.  Like marking peoples' service as no longer active in their online screens as soon as they find out they're moving.  Then the people don't call in to get a confirmation, and months later they're faced with a bill.