I'd guess that anything done to force a change would just result in the horde logging off or quitting the game.
I'm afraid this also makes players on the receiving end logging off.
This has EW arena vibes all over again. You log in, see yourself facing a mini horde stomping all over your bases. If you try to defend, you are quickly overwhelmed or (which is even worse) they spread out and prep several bases at once, striking where you are not. I guess the only good thing is cross county direct messaging being disabled these days, for I remember these guys going ballistic if you actually foil their attack once in a while, or they start gloating about their success...
But alas, I have no real fix for that. It's basically a number's problem, the lower the overall population, the more extreme the imbalance can get.
Maybe stop playing 24 hours a day.
Their fault playing at the wrong time! Everybody should play at US prime time only, no matter where they live or what their schedule is

(For the record, 30 players like in the picture above means Euro late afternoon/early evening these days)