In another thread (Is there a way to beat the new CvC cheat system) a player recounts their clan being drawn against a clan of comparable influence (900 billion) but finding that by the time the match was under way their opponents influence had grown to 3 trillion.

Points scored by late joining members do not count but the presence of the high influence late joining clan hoppers still affects the outcome.  For example their presence makes possible taking the PoP (and issuing both positive and adverse reputes), and makes defending a siege or interfering in the opponents' siege much easier.  In the CvC contest in the thread referred to above the clan who introduced clan hoppers were substantially outscored (152 billion to 182 billion) but won the event six to three.

My suggestion to neutralize this cheating is to introduce a rule such that if any new player joins a clan after thirty minutes before CvC is announced that clan can no longer score points in any of the kill categories.