To whom it may concern,
I am honored that you have considered me for this award/nomination/appointment/coauthorship/[appropriate term here], and take great pride in being a name mentioned in the discussions surrounding this occasion. Truly, I am in great company.
However, I can not in good conscience accept this award/nomination/appointment/coauthorship/[appropriate term here]. The reasons for this ought to be obvious, as your efforts at due diligence should have indicated them in extensive detail. I wish you well in your search for a better suited candidate.
Sincerely,