I want to attack Saxony as Austria, but I can't, even though I have 100 influence in Saxony, the button to remove Saxony from my SOI is grayed out, without any reason!
In 1.2, it seems that one actually CAN remove a country from sphere of influence. There is a 'remove from sphere of influence' choice in the menu. I have not tried it myself, so I am not sure if that's only for removing other countries' influences or also your own. If the former is the case, let some foreign power accumulate influence and they will remove you eventually. Then, it's a question of attacking before the foreign power accrues enough influence to add your target to their sphere.
You could "tag FRA" to france, set influence of saxony to priority 3 and hope when you switch back to youor country they keep at it.
You can usually shake the high-contention nations from your sphere (like Brazil or China) but small, weak uncivs (like Cambodia or Bali) can become stuck to you like herpes.
This also makes going after some tiny nations difficult if they're sphered by someone like Britain. Whoever the owner is will pay attention to you (usually) and make it very difficult to weasel nations from their control due to lack of competition. With coveted nations, you can always rely on the AI nations knocking each other out and wasting influence while you sneak in after they're all broke.