Let me tell you in case of casual vacancy of auditor there is no need to accept resignation as vacation is caused by operation of Law and not by activity of Auditors. However please find the letter below and modify it according to your need.
To,
M/s ABC & Associates Chartered Accountants Lucknow (UP)
Sub: Acceptance of resignation due to dissolution of your firm
Dear Sir,
This is to inform you that your resignation from the position of statutory auditor of the company due to dissolution of your firm of the Company, be and is hereby accepted with effect from the ____ day of the June, 2008 as requested by you.