How to write a professional email to POI to reject an offer?

[Stickyknow]

I need to reject an offer, I wonder how you guys have done it to let the POI know that you are sorry that you can't accept the offer and have decided to go somewhere else?

[Jay's Brain]

One way is to be formal and concise - thank them for their offer but you have chosen to accept another offer. It may seem blunt, but it's also polite and you are not under any obligations to give more than that. Depending on how you felt about the program/POI, you can choose to explain a little bit more. Perhaps the decision was swayed by competitiveness of funding package, location, research interests, etc. Either way, just as you may have received offers and rejections, the POIs also have and it won't be the end of the world for either of you. I have done both, choosing the former for a program that was ranked lower on my personal list and the latter for a program that was ranked higher in my decisions. Both were collegial and there was no concern. 

[PsyDuck90]

Be sincere, but concise. Something like, "Thank you for your [generous] offer. However, I am unable to accept [add more detail here if you feel necessary]."