Is the hadith narrated by Ibn 'Umar (may Allah be pleased with them both): "Used water, if it reaches two Qullahs, does not become impure" authentic, and what is the meaning of this based on the principle of "deterrence is stronger than removal"?

The meaning of the rule "Prevention is stronger than removal" is that preventing something before it occurs is easier than removing it after it has happened. An example of this in the context of purification (taharah (definition)) is what Al-Zarkashi mentioned: if used water reaches two qullas (a specific volume), it does not become considered "used" according to unanimous agreement. This is because using it while it is at the volume of two qullas prevents it from acquiring the characteristic of being "used," whereas collecting it (after it has been used) would remove that characteristic, and prevention is stronger than removal.