Thanks for the replies. I've had to use a direct approach with some of the girls. I tried "I'm not looking for anything serious" and some of these girls didn't take the hint, saying "That's fine, I'm not looking for anything serious either" and playing really cool, but then pestering me to meet up again.
One girl sent me about 4 emails in one day, just to tell me how she understands and how she isn't the pushy type. Then she sent me a text to ask me to meet the next day, or the day after.
I used the "I'm not feeling it" rejection with one girl, and she started asking what she had done wrong, why I liked her at first but not any more. She even started apologising for anything she did wrong. It was so sad.
That said, some girls have been really cool and just backed off. It's girls like this I could gladly hook up with again sometime.
My conscience has always got in the way of me having no-strings fun in the past or even taking chances with someone. I really hate that guilty feeling I get when dumping/rejecting someone. However, it definitely helps me understand the dilemma women have had when rejecting me back in my AFC days and how my clingy behaviour or indignant reactions made me look pathetic.