I agree with collegegrad. There is no need for a reason beyond NGBNFM (Nice Guy/Girl But Not For Me).
If, in a later discussion, a shadchan that I am comfortable with wants to know more details so that the next shot can be closer to bulls-eye, I might consider doing that.
But usually, going into details, ends up making you sound like you are badmouthing the person and there is very little that can be said that will not be taken offence with by the other side.
Another reason not to give a more specific reason is because as soon as you do, (whatever the reason is) the shadchan will try and persuade you that you are wrong and that you should go out again.
If a shadchan is pushy with demanding a reason- you need to just keep on repeating that you can’t put your finger on it, but you are 100% sure that he is not for you.
It’s a shame singles are not ever entitled or encouraged to go with their gut instincts… but that is another story….