I'm thinking of pulling flowers from in front of the house next door and throwing them in the trash.
Throwing flowers in the trash? That's destroying nature. Why are you planning to do that?
I'm going to pour hot water over the flowers before throwing them away
Tearing up someone's ornamental garden is vandalism and is wrong since it damages that person's property and prevents them from enjoying it. Additionally you should try to treat your neighbors with respect and courtesy since you live in close proximity to one another and neighbors should help each other out. If you vandalize your neighbor's garden you will ruin your reputation with your neighbors and will not be helped if something bad happens to your home.
Who cares? My neighbors are complete dicks and they deserve what they get
I believe it isn't a good idea to try and take revenge this way, because the city can get involved and have you fined for destroying property. If your neighbors are not pleasant, why not just ignore them or try to get to know them better over a BBQ?