I really fell sorry for women because there are so many fvcking clueless men out there. I have women reject me HARD at times, because that is what works for them. I don't blame women for this, many men just do not get the message until you chop off his d!ck.
My advice is to start off being polite and CLEAR that they are being rejected. "I am very faltered but I just am not interesting in a romantic relationship with you". It is likely he will say something like "Hey.. who said I wanted any thing serious." Tell him "GOOD, then we are both on the same page." If a guy persists... then escalate your harshness.
DO NOT apologize for how you feel. HUGH mistake, women will say stuff like "I'm really sorry, but I'm not interested." Don't do this, this implies that there are reasons why she is saying no that can be fixed.