This letter thanks someone for giving you advice, but lets the person know that you won't be following it.
1234, Main Street
Boston, MA 02123
07 April, 2005
<Recipient Address Goes Here>
I've been giving some thought to our recent discussion about <topic of discussion>. While I appreciate the advice you offered me, and have given it thorough consideration, I've decided to pursue another avenue just now. I don't want you to be hurt that I did not follow your advice, but I really feel that I would be better off <your solution> at present.
I wanted to thank you for your assistance and for the very good suggestions you made. I will certainly bear them in mind should my own solution not work out.
encl: <List of enclosed items goes here>