How to let down a girl you're not interested in seeing for a 2nd date?

I'm a bit inexperienced with dating so I need some advice on something. Say you go on a date with a girl, the conversation flows well, she's generally nice but its just not someone you see yourself going any further with for a subsequent date, much less a relationship. How do I let her down? Just not call/message her any more after that first date and thats it? (these are girls I meet via online dating, not people I know through friends or colleagues, so highly unlikely I'll ever run into them again) Also how should I end the date with someone like that? Say I enjoyed the time we had but I don't see it going anywhere, I obviously won't be going in for a kiss but I'm just not sure what to say because I don't want to lead the girl on saying something like "I'll give you a text/call" or "Are you free next ....." because those would be flat out lies. Just thank her for a good time and say good night?

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