Actually, I don’t believe they are optional. Both players are supposed to be on the lookout for convoy disruptions and point them out. If you don’t want to disrupt the convoy, you have to move your ships away.
Krieg is on vacation for a lil while and asked Calvin and I to answer questions while he’s gone. If Calvin and I are not in agreement, then I guess you have to wait for Krieg. Or, if you look back through the FAQ thread, you may find the answer to your question for sure.
But I am quite sure I remember Krieg clarifying that they are actually not optional. If an enemy ship is in your convoy zone, it is always disrupting.