This is a slippery slope. We mainly attend parties at a local club to meet folks While there we have had the need to turn down a few people or at least turn them down for "now". Most get the hint but how you do it without upsetting the other party or encouraging them to keep trying is the question. I mean do you tell someone "We don't like sloppy drunks and honestly even had you not spilled over half your beer on my foot without noticing the answer would still be no." or 'The night is still young and we have others we would like to talk to."? We usually try to be gentle and used the second in order to get away from the sloppy drunk who sadly kept coming back even though her husband did not. In fact it took stating bluntly "We are not that attracted to you and your husband and our interests are not the same in regards to what goes on in the bedroom.' We had already mentioned we were not into BDSM and she made it clear my wife's ass begged to be spanked in her drunken state. BTW even then it took us declining a second invite to a house party saying we were not into Karaoke and drinking before it seemed she finally got the hint. It took two more parties at the club before she even said hit to us again. Her husband was as friendly as always and I think understood things. Mostly though it seems the simply a "We need to go get a snack, speak to someone, or just want to go see what is going on somewhere in the club." works and folks tend to move on finding someone they click with. Of course that also leaves thing open to a second "chance" as it were and we have been thankful that happens because there have been times we got to know someone a little more and changed our outlook. Of course there is always (like the last party) times we were not at the top of our game so to speak and while we had fun simply relaxed and chilled that evening with both of us having a backache and being a bit on the tired side of things.
The bigger issue though is how does one turn someone down without making a scene in a more public setting like say over coffee, dinner, or drinks at a bar. The answer is it depends on who you are turning down. Simply put you cannot know how someone will react to being turned down. Sadly most folks think "If we are meeting up then it is likely we will be having sex later." The key is to make sure everyone understands this is just a meeting like say an interview if you will. Nothing will likely happen at this meeting it is just to get to know one another and then we will need to talk with each other before we set something up for the future or not. This means that you can end the evening on "We had a good time and will be in touch." and then the next day send the "We don't think it is going to work" message. Alternately you can excuse yourselves for a few moments then come back to the table with "We talked and if you are interested we would like to take things further in the future this evening." Of course in either case you cannot be sure how the other party will react but with the first response it hopefully delays the negative public reaction, the second well depends on how they feel about it. BTW what is in "" is only a suggestion and you can build on it working up your own way to say it. The key is to fend off a negative public reaction because depending on the other party it could be negative (at least it is just through messages hopefully) or gracious and not knowing which means you have to control the when.