I generally say,
"I appreciate you asking me out, but I'm not interested in taking this any further. Good luck in your search."
"Thanks for texting me, sorry I didn't get back to you sooner. I've enjoyed getting to know you, but I'm just not "feeling this". I wish you good luck in finding someone special".
^^^or some variance of "Thanks, but no thanks"me...BS, 43 years old, 2 small kids
WS, 41, multiple gay affairs
M 15 years, together 17
"For whatever we lose, like a you or a me, it's always ourselves we find in the sea" ee cummings