"The Christian Player may not engage in naval movement in the Mediterranean this turn. may be played to cancel both the Venetian and Genoese fleet cards in addition to the above effects."
If played as event does it prevent the use of those events later in that turn, or does it just prevent a player from playing the 3 ops card to move 5 units?
My interpretation is that you may cancel the Venetian and Genoese fleet cards the moment the Christian player plays them if you have Commercial Rivalry in your hand. The words "both" and "and" suggest that if you cancel either of those cards, you actually cancel both of them. I see two different situations playing Commercial Rivalry as a response event:
1. Played against regular naval movement(since it's a response event)
result: The Christian player cannot make naval movements in the Mediterranean by regular means this turn but he may still play events that allow him to make naval movement in the Mediterranean.
2. Played against naval movement with Venetian or Genoese fleet events.
result: Naval movement is cancelled. Both the Venetian or Genoese fleet events don't work this turn and in addition the Christian player cannot make normal naval movement.
Historically I see two different situations:
2. Rivalry between Venice and Genoa.
1. Rivalry between some other parties.