Plundering for gc while preparing for war is always risky. Not sure if you mean you had an agreed war or anything, but I know some kingdoms if they get hit in any capacity from a kingdom tagged war agreed, or that are in a hostile, etc they will be a lot more unforgiving in their retaliation.
Also assuming that you treat a top 5 kingdom's "inactive" the same as any other inactive on this server is wrong. Usually those kingdoms have a very close knit group of people. Perhaps that person didnt log on in 3 days because he was just stockpiling gc and made enough food etc that he didnt need to cast self spells, so really logging in wasn't required because he can find out "what's happening" just through their kingdoms' chatting tools. So maybe the province wasn't as inactive as you thought.
Either way, If you want to shave some gc from a top province, or from an inactive, the best way to do it is with robs. Get your T/M's to get some robs going. That way the kingdom doesn't see that you took the gc, only the individual province (which as you say is inactive anyway).
Lots of missteps here, and just take it as a learning experience. Or hit them back, whatever you want to do.. that is the beauty of this game lol. Any province and kingdom can do whatever they want, there are simply repercussions for doing certain things. Teach that to your new players, or tell them not to hit out without consulting one of your leaders first. A probation of sorts until they learn the game better.