I already answered it in other posts if not directly responding to you, but if you want another reason, why should I completely destroy the already fragile diplomacy in the game, take a warmonger hit and have everyone that was friends with Venice hate me, when my civ has done nothing wrong and Venice is being the aggressor?
Because that is the cost that has been required to protect yourself from Venice's UA.
Rather like 'having to build pikes' is what's required to protect you against powerful mounted UUs like Companion Cavalry.
Again, you are asking for essential immunity from an entire Civilization. There should be a comparable cost.
Since you are more concerned with late-game than early-game, you should strip Venice of its friends before you put up your War Wall.
First, you MUST prioritize gaining control over the World Congress. First thing you should do is embargo Venice. Give ANYTHING you need to in order to accomplish this. While the new diplomacy system is as yet imperfectly understood, it seems clear that having trade routes with other Civs helps your relations. This would imply that embargoing a Civ (in this case, Venice) will hurt their relations, or at least not help them.
Next Congress, Embargo all CSs. Pass these two, and Venice can trade with absolutely no one. You've just neutralized half of their UA (double trade routes), and nerfed the other half (buying in Puppets). (Note: The MoV Acquisition is a part of the unit, not the UA.)
After that, pass the resolution that promotes growth of GWAM, and penalizes growth of GSEM. Oh hey, look, Venice isn't looking too good now, is it?
Basically, there are any number of retaliatory
diplomatic tools at your disposal against Venice. In fact, I would argue that they are the weakest Civ in the game to this type of Diplomatic attack.
I will also point out that, if you're going the Diplo route, you should have Patronage, thus unlocking the Forbidden City, and should make a priority to build it.
These are all late game answers to Venetian CS Acquisition. Which of them do you think are inadequate?