Caesar was very clear about this in his own writing on the Gallic War - a great "first person" source answer to your question. In his 10 years, he sometimes fought tribes three or four times. He always talked about mercy first, then being tougher, and then being real tough. Tribes who rebelled too many times often ended with executions and mass sales into slavery.

The Averni tribe actually did receive mercy, because they mostly tried to work with Caesar; Vercingetorix went against the cautious nobles in the tribe (who expelled him) and then raised his own army and went back and conquered. So he was a personal threat to the potential internal Gallic allies of Rome.