Do a couple sentences about the weather, or that crazy water-skiing squirrel you saw on the YouTube. They'll feel a little deflated for a half-hour or so and then it's on to the next profile. She thought she had rejected them! Some might accept your response without another thought, but others might feel upset when faced with rejection. First, you need to be honest with yourself. A wink or something, sure, okay -- no problem.
Write one or two compliments, then move on.
Online Dating Dilemma: Dishing Out Rejection
I would respond to someone if I knew them outside of the dating site. Avoid people who feel "entitled" to another date. Not very many women in our culture are forward enough to ask for the date themselves for better and for worse usually for worse. I have been on the sending side of personalized messages on OKC quite a few times. I ignored it because he was so very much not my type physically that it would be an impossible gap to breach, many of his OKC answers were diametrically opposed to mine including the fact that he wanted kids and I do not, which is dealbreaker territory in your 30s ; besides, we did not actually know each other at all. Best of luck in your search.