Android phones, apps and tablets

While we're on the topic of signal, it's great and all, I use it, stuck it on my wife's and bosses and the work phones too. But, we've got a massive shared work Google address list, which also has a load of people who have tried signal (turned off the annoying notification eventually) one of those people (that I know of) stopped using it but signal still tries to send them messages through its system instead of sms unless you explicitly tell it not to each time without any kind of error message. This means the customer just doesn't get texts unless you realise the problem and there's no easy fix, I've tried deleting contacts, reseting stuff and starting again but no luck. It should of course be an easy thing that happens automatically once the other person deletes signal, or there should at least be an sent message error and easy "unsignal" contact button.