If you already had other plans, you should not drop your other plans just because a guy says he wants to hang with you. You should inform him that you have other plans. However, if the other plans came after you made plans with the guy, it would be rude to change those plans after the fact. (One exception might be, "Hey my friend from out of town, that I have not seen in years, just called and said that she is going to be passing through on that night. Would you mind rescheduling so that I can see her while she is here?")
Short answer, no. You should not always be available, but you should be honest about your availability. Even if saying no is just because you don't want to say yes. That is your right as a person. If he can't handle it, you don't need him.
No, women have a busy life's as well.. just like some of the guys do, it is my opinion that you should not drop everything because some guy says.. hey I am coming over to pick you up.. be ready.. but one must also be honest in a relationship both to themselves and to the so called other person.. if you are busy and he or she is cramping your style.. step up to the plate and say so, if his feelings are a bit hurt well so be it he will get over it.. there are 7 days in a week and 30 to 31 days in a month.. just make it for another time
Only say no if you really have something going on and can't keep your date. Otherwise, you are just being stupid with whether or not you should be available or not, like it's a strategy or such. I don't see the point in saying no if you don't have a reason to keep the date.
No. Men don't want you to fucking turn them down you dumb shit. I hate women who think that men are attracted to agressive "I'm better than you, I don't need you" behavior. This shit will either make him feel sad, or it will piss him off.
Make yourself available, show him that you care about him. Men aren't attracted to "strong women".