As I said, if you have the premade holidays in your game, you will need to edit them. You go to your calendar and click the pen icon on every single holiday. That lets you edit the holidays. There should be a check box reading "holiday decorations" somewhere in the upper right corner (near the "day off" checkboxes iirc). Now you uncheck that box.
That was the first step.
The second step is to create a placeholder holiday at the earliest day the game allows. Set a single tradition, for example cleaning. Make sure to have the "holiday decorations" box UNCHECKED. Wait for this holiday to run its course and afterwards the neighborhood should be free of Maxis deco.
Now the bad news:
You will most likely have to play this placeholder holiday in every affected neighborhood. It's a pain, but unfortunately the only reliable way :(