A：给一个比较通用的，美国人也比较乐意接受的拒绝offer信的模板：

Dear Graduate Admission Committee:

I regret to inform you that my parent/friends/BF/GF/classmates/etc. have recommended to me that your program not be admitted to my further study.

This action was based on a careful review of all the materials in your offer. I regret that we cannot accept all Universities who give an offer, but I am aware of the importance of admitting only those programs I feel will be suitable for me and whose interests fit well with my research interest.

I were also constrained this year by an unusually high number of offers coupled with only one I could accept.

Thank you for your interest in my application. I wish you the best in your continuing admission.

Sincerely,

xxxx

当然，这里还有另外一个模板：

Dear Prof.XXXX,

Thank you for granting me admission into your department. Your department is among the best in the U.S. I was honored at the committee's decision to accept me. After careful consideration, however, I have decided that I will not be attending XXXX. I wanted to let you know as soon as I was sure; I hope you will find students who are also very suitable for your department.

Best wishes