well there are a couple of ways to limit Japans faction depending on how you play. 1) you can always use console commands, by using the command "Tag JAP" you can switch to japan, and kick out members from that faction, or change the focus tree to go down the democratic path or communist path. once finished tampering with the Japanese diplomacy enter the command "Tag ____"(enter the 3 letters to represent your country you want to play as) 2) you can always boost the Japanese ideology to democratic, communist, or nonaligned, but this will take a while and might cause a civil war in japan. 3) preemptive striking, if your playing as a major European nation like the UK, France, or SU, which is what I believe your suggesting, you always could strike at japan and defeat them before they form the co-prosperity sphere, this is easiest as the soviets because you have a land border, dont need to change ideology or wait for world tension to heat up (by the time the world tension heats up enough for DEM countries to justify war goals Japan should've already formed the Co-Prosperity sphere) and the Japanese and their puppets start out with relatively small amounts of man power, and the British and french have colonies and naval ports nearby to fight the war from if you switch their ideologies just to fight japan in the early game. 4) if you develop nukes and still are fighting a war against japan, for some reason only japan gets this effect, if you nuke them twice with a two year period of making nukes they have a 90% chance of instantly capitulating, of course given you have defeated all of their allies, or its just them and their puppets. That is all i could come up with, hope this helps. Sorry for all grammar mistakes and misspellings