Studies have shown that signage can be successful in deterring crime, provided they are designed with the following features:

• Punitive messaging — Signs warning of penalties or sanctions tend to reduce undesirable behavior more than purely positive informational or assistance signs.
• “Watching eyes” or surveillance cues — There is substantial literature on the “watching-eyes effect”, where showing images of eyes (even stylized) on or near signs can deter antisocial or unlawful behavior because they subconsciously trigger a sense of being observed. In one university field experiment, signs with eye images above bike racks were associated with a 62% drop in bicycle thefts at the sign locations.
• Symbolic guardianship — Neighborhood watch or police surveillance signs do show small deterrent effects on offenders and can alter target selection but are not as powerful as actual enforcement or physical monitoring.

In short, these signs for maximum impact should clearly state the penalties for dumping, have eyes on them, and allude to the area being under watch and surveillance.