Bismillaah.
Sister, if you feel as if you do not wish to marry him, or get to know him at all - then you need to learn how to say NO!
Now, without even knowing it I am currently raising my eyebrow. I myself am not as assertive as my fellow sisters who will kindly tell a brother to get lost. Before I get refuted, I'd like to emphasize that I am not encouraging sisters to say no to every single Tom, Dick and Ahmed who approaches them for marriage. No, not at all. I am teaching my lovely sisters (and myself) that actually this is how you tell a brother. 'NO! No akhi, may Allah bless you, I have prayed istikhara, done my research and received advice therefore I am not looking to be your wife. Thank you'. Maybe the capital NO with the ! on the end comes across as a little aggressive. Don't be aggressive sister, don't shout scream or tip toe.
1) Be assertive yet calm and collective. Please do not get over emotional when you reject a marriage proposal. Everyone (your family, his family, and him) will think something triggered it, like a horrible childhood experience or something. Be calm yet assertive, you do not wan't to marry him so do not let any one try and manipulate or persuade you into it if you have made your final decision.
2) Explain why you are not interested. Pretty straight forward.
3) Don't lie. Please. Don't tell him the fabricated story about how you had made plans to marry a cousin living in Pakistan way before you had even met him and that once married, you and your cousin are planning to head to Thailand to become hotel labourers and live happily ever after. Besides it's haram to lie, so don't go there.
4) Show consideration. Don't just be rude or vague about it. Mention his good points and make duaa for him in private. Saying no - inevitably- deflates his ego. Last thing you want to do is deflate his whole self worth and self confidence. Believe me, some men are emotional like that.
5) Bring Islamic evidence. Make it clear that although you have refused him for marriage, you know that you are allowed to reject him i.e your decision is supported by evidences from Qur'aan and Sunnah, as it is permissible for a woman/man to reject someone for a number of reasons.
However I want to stress that denying a brother for ridiculous, superficial, tribalistic, nationalistic reasons is very dangerous and for you to deny a brother there must a really good reason behind for it.
So if the reason behind your choice is..er..reasonable, just say NO! (refer to above post for guidelines on how to say it)