Once you have accepted an offer with an organization you are obligated to notify all other employers to whom you have submitted applications that you would no longer like to be considered for the position in question.  This can be done though a business email.

Sample

Dear Ms. Roberts:

I am writing to withdraw my name from consideration for the Accounting Assistant position with Lakeland Industries.  As I indicated when we met, I have been exploring several employment options.  This past week I was offered an opportunity with another organization, which I have decided to accept.  I believe it to be a better match for me at this time.

I do thank you for the chance to interview with Lakeland Industries and to be considered for employment.  Your organization is an excellent one and it was a pleasure to have an opportunity to learn about it.

Sincerely,

Zhang Wei