Formulate your refusal politely, in no way justifying his personal qualities of the applicant. If you always take care of the people around you, not to hurt them, and you in turn will not hear anything bad about themselves.
Speak firmly, so that men do not get the impression that you are still in doubt, and if it is persistent, you do agree to a meeting. Thus, you can only worsen the situation and bring it to the point of absurdity. Do not fall for tearful entreaties - go to this date, you risk ending his quarrel.
If you have not understood in its relation to the young man, and want to pause to consider further actions, say so. In any case, honesty and sincerity in relations between people better than lies and evasions. Explain that you want to quietly reflect on the situation and do not want any pressure from the outside.
Often real, but not serious at first glance reasons prevent you go on a date with her beloved. There is also no need to lie and invent something more impressive. You can get tangled in their own fairy tales, and the man caught in a lie, loses confidence. Even a minor inconvenience that can ruin your date will be correctly understood beloved, unlike unrighteousness.
Transfer the meeting at a suitable time when nothing prevents you to enjoy each other's company. Together think and assign the time that suits you both.
In any case, do not settle on a date with you nasty man who does not inspire confidence. Communicated to him the meaning of your failure and unwillingness to hear such proposals in the future.