In R2, the second season of Code Geass, Lelouch was already under the Emperor's Geass so he lost his memories related to Marianne, Nunnaly and Zero. But when CC kissed him or something, she managed to undo the Geass and make Lelouch recover his memories. That is what I find confusion in, I mean, by definition CC is able to make Geass contracts, be immune to another person's Geass and she is immortal. Also she is able to use the Code's power to connect to the world of C. So... how can she do that?
Lelouch's power is "the power of absolute obedience"
Mao can hear the thoughts of anyone within 500 meters which is refereed to as "the seal of absolute sound"
Rolo can create a sphere which freezes people known as "the seal of absolute suspension"
while these are the only ones where their powers are actually named we can assume that the power of Geass is Absolute, it's not explained how a Geass power is absolute over one another, as all Geass Uses used a different power and when we see them used on Lelouch they affect different parts of him (Rolo's affected one's sense of time, Mao could read his mind, Charles seemed to be able to manipulate his memories)
Rai from Lost Colours had an non optical version of Lelouch's power however he never (to my knowledge) uses it on Lelouch, Shin Hyuga Shing's Geass seems like Lelouch's however it seems to more relate to people dying and we see him use it during Akito of the Exiled, when Lelouch's Geass was sealed (so even if the 2 met we wouldn't know how Lelouch's Geass would react due to it's sealed state)
Given in most cases Geass seems absolute apart from some weakness like Lelouch and Rai can't using it on people a second time or Rolo's heart stopping, the Geass Code which is what C.C and Charles has would be absolute aswell, since all Geass Powers come from a Code barer and a contractors powers have no effect on them (we see Lelouch try and command C.C and Charles) it could be that they are absolute over contractors, Lelouch even argues with C.C she could have terminated her contract with Mao but he may have been referring to killing Mao rather than stripping him of his power.
Something which may be able to back this up is Jeremiah Gottwald's Geass Canceler which was developed by the Geass Order. C.C was it's previous Director followed by V.V before finally Charles, Charles's Code came from V.V so all Directors of the Order had the Code, Bartley also mentions at one stage that they captured C.C some time after she left to study her which could indicate that the Geass Canceler may have been an application of their studies of the Code given the order would research the Code first in order to duplicate Geass Contracts
i should point out this is mainly just speculation from what i've seen in the series, there are a lot of mysteries to Geass which have yet to be revealed.