Recruiters are trained to keep pursuing a person whose initial response is no. They are trying to sell you on being a marine, and will persist until it is clear that you will not be joining. There are ways to be clear and polite while still being polite. If you have specific reasons for not wanting to join, explaining them to the recruiter could be helpful (although he or she might try to talk around those reasons). Also, being clear that you have goals for your future and that those goals do not include being in the marines can be helpful. Ultimately though, if a recruiter is persistent, you will have to ask him or her to leave you alone. You can say something like "I'm sorry, but I'm not interested in joining the Marines. Please stop trying to recruit me."