Keep in mind that in reality the employer is not too interested in the reasons why you rejected the job. However, be prepared that they will ask you why you did this.
Do not Express "the bitter truth" to the employer, even if it exists, and you think it's necessary to restore justice and to justify his refusal. In such a situation, cut to the "live" the employer is able to start a "war" against your "baseless" attacks. It, however, it's easy to understand: you took him a while, and then decided to give up.
Not motivate their refusal by unwillingness to pass the tests. Maybe you find this procedure too prolonged and fundamentally biased. Representatives of the companies operating in this area are more likely to think in a different way, relating his work to a number of significant achievements.
Take the example of employers. Remember how they explained his refusal after an interview or testing. Do not reply in monosyllables, but at the same time use the minimum of words. For example, your "I'm sorry, but at the moment I can't accept your proposal," will be the most appropriate phrase. Don't try to clarify. Say your speech in the most correct and friendly.
Don't worry and don't panic. Of course, it would be better if you are intelligently and briefly summarize the reason for your denial and present it in plain language, without taking time neither himself nor the employer. Create, record and learn a short phrase, and most succinctly describes the reason for your refusal. Refer, for example, the inconvenient location of the office or on the characteristics of the proposed payment, if such is indeed the case.
Do not worry, saying to the employer "no." Remember that you are just another candidate for the proposed position. You can and should choose the best option for themselves.