Well it seems that the general consensus is to disable or ignore the alerts and just get on with life, and I expect that's what I'll do. But to those that pointed out that port scans are a fact of life, yeah, I get that. I didn't come down in the last shower, and I know it's a big bad scary world out there, but the UTM is intelligent enough that it only raises an alert when a scan is considered particularly egregious. Even with all the script kiddies and other scanners out there, I get an average of less than one port scan alert per week under normal circumstances, not counting the one routine scan that I myself have requested. So when I started getting multiple reports daily, every day, from the same subnet, yeah, it got my attention. Analogously, we get people ringing our front door bell once every couple of weeks, but these folks are standing on our front stoop ringing that bell all day every day, and it chokes my goat to just shrug it off and let them keep doing it.
Turning off any alerts goes against the grain, but as y'all have pointed out, as long as the defenses are in place then stuff bouncing off the walls doesn't really warrant concern.
To those that suggested filtering the alert messages, I have considered that, but I don't currently have any means of filtering based on anything but the mail headers, and the originating address only appears in the body. Still, I may look a little further if I start to twitch because I'm "missing" alerts.
To those that pointed out that the UTM ought to be filtering before detecting, yeah, I get that too, and in fact I have raised it with Sophos, but unfortunately as a non-paying Home Use customer, my voice doesn't carry a lot of weight. I do get that I could probably cobble something together using Open Source and a bunch of cryptic incantations, but frankly, I do enough low-level stuff in my day job - when I get home, I just want to enjoy my internet connection, not spend hours maintaining it. But thanks for the suggestions.
So in summary, I guess it's time to turn off the notifications, stick my virtual fingers in my ears, and start chanting Merry Christmas. Cheers!