1） I am sorry but I just accepted another offer that I feel is more suitable to my needs / more right for me./ more interesting to me.很抱歉，我刚接受了一个我认为更符合我需要的/更适合我的/更感兴趣的工作。
2） I really appreciate your offer, but I must decline it.非常感谢贵公司的录用，但我不得不拒绝这份工作。
3） Frankly speaking, I have been thinking that the position is not right for me after the interview.坦白地说，面试后我一直认为这个职位不适合我。
4） I really think that position suits my education background better.我真的觉得那个职位更适合我的教育背景。
5）I don't want to accept a position that I don't think I will be happy in/offers me too little salary/ can't supply me with an apartment.我不想接受一份我觉得不会让我快乐的/一份薪水太少的/不能为我提供住房的工作。
Your words should be professional and courteous. You may be turning down this position, but you might want to be considered for future opportunities. Savvy （有见识的） job seekers use every possible chance to thank each person with whom you interviewed and wish them and their company continued success.