Stxman: about the only way to deal with that is to have a large navy at your disposal to immediately wipe out your attackers' ships. The beautiful thing about naval battles is that once they are over, you will automatically go back to neutral status with your enemy as long as your boats can't see them, and their boats can't see you (actually the same with land battles, but easier to accomplish with boats). So, after the Italians attack your navy, starting a war and messing up your trade routes, simply overwhelm their boats, killing all of them. Then retreat away from any ocean sector which borders an Italian province. Next turn, move back into your trade routes. The most frustrating thing will be when the enemy ships run from you rather than stand and fight. Then, you will have to waste a lot of turns chasing them around before you can reopen trade. It's just part of trying to be a peaceful maritime trader in MTW.

Welcome to the org, and by the way, don't be afraid to start a new topic if you have any other questions (this topic was about influence, not naval tactics).