In my experience sometimes it is a little more complicated than being honest or not. Some people take rejections well and other people might become aggressive. So the best approach is to disengage.
I mean, normal people would accept polite rejections and understand their meaning. If you say "I loved to, but I can't" it really means "no, but I don't want to hurt your feelings". But some men would translate that as "I'm playing hard to get" due to social conditioning on "traditional" societies. It is absurd, but we still deal with that. You can try to say "Sorry, but I don't want to", but that is already considered rude.
The bright side is that there is no need to be a mindreader. If you get a polite uninterested answer, you already know that they aren't interested enough to make an effort. So it is better to move on. If they are interested but were playing "hard to get", they'll make up excuses to talk to you again. The only place you can't apply this is work, where people would have real reasons to keep talking to you even when not interested romantically.
But where I live it is common advice to avoid romances and hock-ups at work to avoid drama at work. You might lose your job because of that, so it is not the best idea.
The point is, if someone wants to date you but can't, they would say something like "I'm sorry, I can't on monday but we can go another day". And here I'll add, you can give a couple of tries to set a date. If you can't agree on a day to meet, then that person either has other priorities or a very complicated life and can't. Either way, it's not happening and not worth the effort. And it is on them to suggest a day a plan it since they are the ones with complicated lives. Then if you can and want to, you can go, but you have no obligation at all.
It is better to be with people who show interest or even enthusiasm to be with you. Don't settle for less. And those things show you can notice that. Even traditional women that aren't direct about that would do little nice things and invent excuses to be around you (non-work related XD).
