You need to respect your parent's lifestyle. It may be naïve and stupid, but disrespecting it will not influence them to change.
At least get them to fix the alarm system. You've made a fuss over it, so now you're going to need to give it some time, or you'll appear to them as "their badgering, know-it-all kid".
After about a month, and it needs to be a month, of it going off every week, with you not reacting in any way, try this tactic. Shame him. Not overtly, but subtle. Most men pride themselves on their homes, and their ability to have things work, and making smart buying decisions.
"You know, Dad. Its a shame you spent all that money on a monitored alarm system and it doesn't work properly. It is a good alarm system, isn't it? I mean, you didn't get swindled by some fly-by-night alarm installer, did you?"
<Gruff answer from Dad . . .>
"Oh. Well have those guys come out and fix it. I can't imagine how embarassing it must be for you when guests are over and the alarm goes off at 2am."
That's about the best you can do. Get them to uncondition the response they conditioned themselves to: alarm = false alarm.