I think that it's time that as a society we sit down and agree of one phrase that says succinctly the following:
Trust me, I know exactly what I want, or at least have a good idea, and judging by the way this is going, don't ask why, don't take it personally, but you and I would never work out. It's not you, and it's not me— it's the fact that if we were both in the same equation, it would perpetually be imbalanced, and we would waste a year of our lives trying to rebalance it, only to fail miserably. You're nice, but you've got issues with which simply I won't be able to reconcile. There's someone out there for you, but it's not me. Sorry. You might be a good friend, but it would never be more than that. Ever.
Simple as that. Just one phrase. It needs to be unambiguous and simple to recognize. Perhaps "Zappy the clownbattery" would work. It just sucks when someone tries to talk to you, and makes really shitty small talk to the point where you know, definitely, that the person is either too stupid or too simplistic to ever be able to tolerate for more than 30 minutes— much less date. On top of that, you know with certainty that they'd never be able to tolerate you. So, the only way to convey that is to invoke the Zappy clause.
And, it sucks, because people take it personally and wonder how they can "change" and whatnot, when in all reality they can't. Even if there were some magic personality changing device, it's unlikely that it would have any effect on the already-formed opinions by one person on another.
Anyway, we just need to leave it at that. Someone says hi, someone else detects that you might be hitting on them by the sharp pain in their stomach and the wincing of their eye. Subsequently, they say "Zappy the clownbattery," and then it's mutually understood that that's as far as it goes. No awkward pause or silence; no backstabbing; just pure and simple. Think of all of the broken hearts that would be prevented.
Cheers. :P