Include in your Sailing Instructions a rule requiring conformance with the Clean Regattas program. You may use the below example, or similar language.
DISPOSAL OF TRASHThis event is designated a CLEAN REGATTA by the Sailors for the Sea organization. By entering, all competitors agree to abide by CLEAN REGATTA Certification Level requirements. Boats observed discharging trash into the water will either be disqualified from all races the day that the infraction is observed or be subject to other suitable disciplinary action, or both.