Celestia was kind of a bitch to banish her for 1,000 years, I mean really. A. thousand. loving. years. That's a long loving time, and realistically she would probably have been driven irreversibly crazy because solitude for even a 100th of that time is enough to cause a mental breakdown due to isolation.
The first time, Luna just refused to lower the Moon. Banished for 1000 years.
Second time, refuses to lower the Moon again,
and continuously tries to kill the Mane 6. Banishment?