How to email Graduate Director that I will not attend (F1 student)
Agree - you have to tell Y. I agree with the above but add capitals to Admissions Committed.

Dear Professor X

Thank you very much for the offer of funding to complete my PhD with [name of uni]/your Faculty/Department [match to the Director's level of appointment].    After some thought, I have decided, respectfully, not to accept the offer.

I remain though very grateful for your consideration and I would like, through you, to convey my thanks to the Admissions Committee for their support.

With kind regards,
Sincerely,
ZZ