If it's built, when you select it you should have a little button in the top right corner of the info window, that lets you upgrade the decoration to a version that can be removed.
If you've placed it but it's not being built for some reason (ususally missing materials), you'll have to go through the townhall UI to cycle through the building jobs till you find the right one to cancel it.
It's not a bug, it's a consequence of the sneaky little bit of workaround required to allow decorations to be placed on roads. ;)

Thank you so much, I finally figure it out somehow :)