We learn the ascended won't interfere in the affairs of mortals.
Daniel Jackson ascends with Oma Desala as his guide. Soon afterward, the system lord Anubis appears after what has been a long absence. He somehow self-ascended but the Ancients rejected him. They failed to annul his ascension entirely so he has returned as a half-ascended abomination with more knowledge and power than all of the other system lords combined.
Anubis is about to use this ill-gotten might to exterminate the people of Abydos when Daniel Jackson, in violation of the rules regarding intervention, tries to stop him directly. Oma Desala seizes Daniel mid-strike and carries him away. The Ancients allow Anubis to continue, and later expel Daniel.
The Ancients allow Daniel agency in the mortal realm after they eject him. This agrees with their stated policy because he's now mortal as well.
If Anubis had remained ascended, the Ancients would have tried to prevent him from killing any mortal. If at that time they tried but failed to denude him of power entirely, surely they wouldn't just give up and leave him to continue as he had always intended; the attack would have been the entire reason for his expulsion. But when he attacks the people of Abydos using advanced abilities they had already tried to take from him, they stand aside as if it mattered when he made his plans.
In other words, the Ancients prohibit Anubis' ascension because of his character, but they allow his illegal power to enable the effectuation of his character.
The philosophy here is a bit too deep for me to grasp. Can someone tie it all together -- intent, responsibility, power -- in the context of the Anubis sequence?