Here are some insight from a frequent traveller who relies on internet to stay connected with work (while on vacation 😞 )
I've stayed at quite a few places with internet caps, where I've maxed out the internet after simply checking my e-mail for 15 minutes. The culprit: Automatic updates and background app refreshes.
Most people's iPhones and laptops are set to automatically download updates. A new iOS update can be 1GB+. If the person happens to have Xcode installed (the tool developers use to build apps for iPhones) and an update comes out (usually in paralell with an iOS update), then that's easily another 3GB download. Then when you consider all the apps people have on their phones and all the updates that happen with those, that's easily 50MB+ PER APP download. Add to this that usually when a new iOS update comes out, many of your exisiting apps also get updated in parallel to support the new features.
Then consider that it is not uncommon to have each person staying with you own 3 or more devicices: Laptop, iPad, Iphone, and maybe even an iWatch. If an iOS update comes out that is 1GB in size, and all 3 devices download it, that's 3GB in a single day before your guests have even checked e-mail.
Not to mention that if you open up your Facebook app to check messages from friends, it starts to auto-play videos weather you watch them or not, etc... Plus all the other apps running in the background constantly checking weather, mail, sports scores, notifications, etc...
So consider this scenario: You have 4 tech savvy guests staying at your house, each with an apple laptop, iPad, iPhone, and an apple watch for good measure. Because of bad luck and the universe conspiring against you, apple releases a software update while they are staying with you. All their devices start downloading updates at the same time. In the background. Without them even knowing it.
Total data usage that day: 24 GB
Each device: 1GB system update + avg 10 app updates @ 50MB each = 1.5GB
x 4 devices x 4 people = 24GB
A total of 24GB in usage in a single day. Possibly without any of your guests even touching their machines.
All this to say, that it is very possible that the guests were following your instructions, and limiting their internet usage, and are just as frustrated as you are that they've hit usage caps.
I'm not sure what the solution is. But when I travel, I make sure to disable all auto-updates on my laptop and phone to avoid needless data usage. (Since I often tether my laptop to my cellular personal hotspot, it can be devastating to my data plan if my laptop decides to download a large update). Although not super complex to set this up, I think it's too much to expect your guests to know how to do so.
So currently I don't think you have a great set of options:
(1) Not offer wifi (but then you lose renters like me who need wifi) or
(2) Get unlimited bandwidth plans or
(3) consider blocking access to apple's and microsoft's update servers (requires technical know how on your end) or
(4) continue to eat the overages.
Hope this helps...
Best,
Alex