You should never start in the Ancient era... always set it back to Prehistoric. There are known undiagnosed crashes and it just fouls up the flow of the game to start in ancient.
Could what be disabled? The ability to start in another era? It's out of reach... which is why it comes up with Ancient as a default unfortunately. We've never discovered where we could at least set the default to something else at least.Never tried it myself. But could this be disabled? Or is it part of the core?