Thinking it over, just for clarity's sake, I figure I'd give just a brief example for readers because from what I remember the previously shot down bracket suggestions didn't work because they didn't work like real life's. They would have people of a certain income be taxed all at one rate, which is what leaves room for farming. And I'm not implying the previous replies are wrong, because I'm sure there be some room for what they're saying, but in the end, who'd be in an alliance that farms them? It all works out. Anyway, here's this:
Income Brackets:
$0 - $1,000,000 --------------------- 10%
$1,000,001 - $2,000,000 ---------- 15%
If I earn $1,500,000 per day, I'd pay $175,000 in this system -- compared to paying $150,000 with a flat rate of 10%. This is because my first million is taxed at 10%, and what fits into the next bracket ($500,000) is taxed at 15%.
It is this, that should make farming impossible. Because it doesn't matter what an inactive is making per day, they couldn't be coupled with all the other inactives into their own bracket. They would inherently fill all the same brackets other alliance members already occupy. So the only way to farm, is to farm literally everyone. And like I said, who'd stay in an alliance that farms them? It'd all work out.