i am totally incapable of answering this because i rarely leave my house, let alone have a level of social interaction even approaching this. but i feel like we get asked this at least once a week, and it seems sort of important, so i asked a friend of mine who frequently and successfully partakes in date situations with other humans. she says:
“No one wants to be the person who lets other people down. We’re all pleasers. Which is why it’s so hard to tell people we’re just not that in to them. But you have to do it– the sooner the better. While it probably takes longer to decide if you want to be in a serious relationship with someone, two dates is likely enough time to realize if you definitely don’t. If you already have another date planned, call her as soon as possible. If not, call her as soon as she asks you out again. Be polite but direct and don’t leave her on the hook. Say something like "While I think you’re a fun and intelligent girl, I’ve realized I just don’t feel a spark between us. I’m sorry.”
DO NOT SAY “I’m just not really looking to get involved with anyone right now.” Because that makes them think you might be available to them some day. One final caveat. If this person is someone you have truly only seen twice (meaning you’re not also friends/friends of friends) any you’ve NEVER spoken on the phone and you’ve only texted, then if you’re really freaking out you can text them this. However, a phone call is much nicer.“