Is it possible to cancel an accidental placement of a decoration?

You can't cancel within the game. Maybe if you contact customer support and have a good reason, they can reset your game back. But you'll lose everything you have done since then.

In the grand scheme of things, you can make up 9000 coins in a few days. When you need to come up with 72,000 coins for super high-flower decorations, you'll find earning 9K was a breeze.