How should i stop spitting after using sodium fluoride mouthwash?

After a few times: How long? Assuming you are an adult, ingestion of sodium Fluoride is not critical for you. Rinse as directed, spit excess, and you are done. For children, who are still growing teeth, it is a different matter. Their exposure should be managed. See your dentist to discuss this.

Over spitting: After spitting 2-3 times you can swallow and walk away from the sink and don't think about it. If you don't like the taste, try another flavor or brand.

Messy: I take it that you rinse with the Fluoride and then notice that the saliva seems to increase in flow, and you don't want to swallow the fluoride. Try to wait as long as you can and then rinse with water.