Dear Agent,
I really thought I wanted to work with you, but your request for a partial just didn't hold my attention all the way through. I was hoping for something a bit more existential, a bit less direct, a bit more...not in all lowercase letters. Good luck finding clients elsewhere.
Sincerely,
Already Signed Somewhere Else
***
No, really I would probably say something about how grateful I am for their request, but I must regretfully decline to send the partial because it and I are movin' on up...
Congratulations!!