Majority shareholders can't just vote to take away equity from minority holders. There are laws about minority shareholder rights. The OP better have some really specific agreements in place.
Well generally with more or less standard docs in place, the board can vote to remove the offending person from the company. And then it's just a matter of the vesting schedule as to the leaving person's remaining stake (this is why founders should have vesting). Not saying there won't be something of a legal battle, but certainly there's no special provisions necessary which you wouldn't expect in standard startup articles of incorporation.