in the bottom of the email, just thank them for their time, let them know that they didn't get added to any list, they aren't going to hear from you again if they're not interested and you respect that they are busy and their time is valuable
legally it's considered an auto opt out
and professionally i'd wager it would garner a bit of respect
it keeps you in compliance with the can-spam act
and keeps the potential client from having to worry about recieving an assload of spam from you
and to the guy who said can-spam act doesn't apply to him.
if you live in a country where it's okay to flash your weiner to a 14year old girl, and flash your wiener over the internet at an american 14year old, you get dragged to american prison and tried under american law.
why?
because you committed a crime on american soil.
and your country has an extradition treaty with america
that is why kim dotcom is sitting in american prison right now
even though new zealand law has no problem with what he did
american law does, and his servers were sending copyrighted info to american users
with international communications, if a crime is committed on the recieving end of the communication (according to the country recieving the communication) it is still a crime in that country, may not be in the sender's country, but that just means they're only being tried in one nation (assuming of course that there is an extradition treaty between the two, if you're in america and commit a crime through communication to china, and the chinese authorities come after you, america will most likely give them the finger and tell them where to stick it)
this is not to be used as a substitute for real legal advice, i do not even play a lawyer on tv. want a real opinion pay a real attorney bla bla bla