Left-behind children in rural China are minors who remain in their rural hometowns under the care of their grandparents or other relatives while one or both parents migrate to urban areas for employment, face an elevated risk of online moral deviance amid near-universal internet access, prolonged parental absence, and caregiving by grandparents with limited digital competence. Existing prevention strategies, however, were developed for general populations and typically presuppose co-resident parents and well-resourced schools, conditions that left-behind families rarely enjoy. This qualitative study explored prevention strategies that the adults closest to these children perceive as feasible and effective. Semi-structured interviews with seven teachers and seven guardians at two rural schools in Sichuan Province were analyzed thematically and triangulated with document analysis of the Chinese Ministry of Education's Implementation Manual of the Guidelines for Moral Education in Primary and Secondary Schools. Five interlocking strategies emerged: ensuring adequate teacher provision and professional curriculum instruction; embedding prevention in daily school practices as micro-preventive mechanisms; enhancing guardians' supervisory competence; strengthening school–family collaboration and guardian accountability; and building a protective external ecology through government, community, and platform support. The findings portray prevention as a layered, context-embedded system whose effectiveness depends on enabling the adults, institutions, and environments surrounding the child rather than on changing the child alone. The study extends research on preventing online deviance to an overlooked population and offers practical implications for rural schools, guardian training, community provision, and platform regulation.