The New York City Department of Sanitation regulates objects placed on NYC sidewalks & streets:
Sidewalk areas must be kept free from any obstruction that could impede pedestrian traffic. Obstructions may include, but are not limited to, refuse, refuse containers, merchandise, bins, racks, coin operated rides, sandwich or A-frame signs and overextended sidewalk displays/stands.
It is illegal for anyone to place or leave any moveable property (regardless of ownership) upon any public street or any public place. This law also applies to large construction or demolition containers which may not be places on the street without an appropriate permit from the Department of Transportation.
Placing items on the sidewalk or in the street illegally could get you a fine up to $300.
The Department of Sanitation also enforces the rules against posting of illegal signs on public property:
It is illegal for any person to paste, post, paint, print, nail or attach or affix by any means whatsoever any handbill, poster, notice sign, advertisement, sticker or other printed material upon any curb, gutter flagstone, tree, lamppost, awning post, telegraph pole, telephone pole, public utility pole, public garbage bin, bus shelter, bridge, elevated train structure, highway fence, barrel, box, parking meter, mailbox, traffic control device, traffic stanchion, traffic sign (including pole), tree box, tree pit protection device, bench, traffic barrier or hydrant or other similar public item on any street... Every handbill, poster, notice, sign, advertisement, sticker or other printed material shall be deemed a separate violation. Anyone found to have violated this provision, in addition to any penalty imposed, shall also be responsible for the cost of removal of the unauthorized postings.
You could receive a fine of up to $200 for each instance where a poster is traced back to you. Second offenses can cost you up to $300.
The fine for illegal posting on street trees can be as high as $200 for the first offense and $550 for subsequent offenses.
If you notice an illegal sign, call 311 or report it online at DOT's website. Be sure to write down the location of the sign and any information that identifies the party responsible for posting it, such as a business name and phone number.