That's fair enough, refuse to send an arrival message but offer to cancel the flight plan.
After a couple of months/next time they fly they will just cancel instead.
The phoning the helpdesk is a bit off a pain in the bum to be honest poor signal at farm strips and other such things. When they can get a signal they are driving and by the time they get home they are over due.
If you could only send your arrival message by text that might work. Although there would be no guarantee the aircraft was on the ground when it was sent.