I had similar experiences, except I have had a couple individuals contact me about an "open position" or a "job opportunity." I just responded with something like, "This sounds like a great opportunity, in which I feel I am highly qualified. However, I would like to make it clear that I am not looking for employment. I am a business owner who is looking for clients that can utilize my unique abilities. To find out more about me and what I do, please feel free to visit my
web site. If you feel that you could use my services, please contact me via
e-mail or phone to discuss your specific needs. I look forward to hearing from you." Then I make sure to have all of my contact information in my signature line, with my business name in
bold.
I don't know if that is the best way to respond, but one individual e-mailed me back and the other one didn't, so I'm 1 for 2, so not too bad.