Recommendations

I've been asked to write a letter of recommendation for someone with whom I've had no contact for over five years. He/she didn't ask me him/herself; rather, s/he submitted a list of names to a third party, who is now soliciting the letters.

For unbloggable reasons, this isn't a letter I can write. Let's just say that this falls into the "If you can't write only positive comments, don't write anything at all" category.

It's not for a student, so I can't use the "I didn't have you in enough classes to get a good sense of your abilities blah blah" excuse.

Suggestions?