You don't owe anybody an excuse. "Just say NO."
I don't feel obliged. I genuinely sympathize with a small amount of them, if I'm in a good mood or feeling especially altruistic I'll give 'em one of those excuses. If I'm havin a bad day or it's the same guy that's always "out of gas on his temporary visit from Chitago" then they're lucky to even get a NO. lol