So many people I know seem to think the kindest thing to do is ghost and/or make excuses if they aren't interested in someone.
The thing is, by doing this the rejected person is still going to understand you aren't interested, it's just going to waste a bit more of their time and possibly lead them on.
Aside from a small number of people who will react angrily, the majority of people aren't going to melt if you tell them you don't want a second date, don't want to take things further, don't see them in that way.
You don't have to be rude or mean about it. If you tell them you prefer to be friends/don't feel a connection or whatever then yes they'll be disappointed for a bit.
If you tell them you're busy/a lot going on/something came up/ghost. .. they will also get the message you aren't interested, however they might first take your 'busy" message at face value and try again later down the line/wait for you to get back in touch/analyse your message.
I just find it a bit sad that so many adults do this. About a year ago my friend was rejected by a colleague who made a big deal about how he doesn't date colleagues. 2 months later he was seeing a different colleague.
I just think if someone's taken the time chatting to you, meeting you for a date and so on then you need to not be so cowardly and actually be upfront.