Waste disposal at undesignated sites in public open spaces causes hygiene problems, city landscape deterioration, and urban flooding in many developing countries. We used different types of norms—subjective norms, perception about government pressure as one injunctive norm, and personal norms—as interventions to promote people’s intentions to avoid disposing of waste in public open spaces. Six different statements focusing on these three norms were designed, and the residents’ tendencies to follow each statement were investigated using 15 pair-wise comparisons in Phnom Penh, Cambodia. In addition, the likely effectiveness of each statement to encourage the residents’ intentions was asked of national and local government officials using the same pair-wise questions. The results showed that residents were most likely to follow an intervention focusing on personal norms, especially when touching on people’s responsibility, and this was also perceived to be the most effective by the government officials. Social pressure from the government, either in a strict way with punishments or in a soft way with recommendations, were the least preferred and thought to be ineffective by both the residents and government officials in avoiding waste disposal in public open spaces. The result suggests that future policy implication should be more focused on bottom-up approaches rather than top-down counterparts. Voluntary civic engagement is more vital in dealing with waste disposal in public open space than government interventions.