Demonstrate proper chair use to your class. Point out that the chair should have all four legs touching the ground at all times.
Ask students whether they think that tipping the chair back is a safe practice. This engages the students in the activity. Point out that it goes against the "Be safe" rule that you have probably adopted for your classroom.
Develop a new classroom rule about chair tipping, if necessary, using positive words. Some students will understand that chair tipping falls against the safety rules of the classroom. Others will need a more specific rule. State the rule in a positive way -- say, "Keep all four legs of the chair on the floor" rather than, "Don't tip the chair back."
Explain the consequence for chair tipping. For example, a student caught chair tipping may lose his chair for the rest of the day or may lose another type of privilege, such as sitting out for five minutes during recess time.
Enforce the chair tipping rule whenever you notice a student tipping his chair. It may take time for students to stop the behavior.