Options:
1) say "no" and do not elaborate.
2) say " I've seen how you have no regard for the property you are renting and don't want to rent to you.
3) random lie, "already have a few interested folks" or whatever.
I wouldn't even let them apply, I've turned down people on the initial phone call because of things they mentioned ( hoping to get by not working all winter). You don't have to tell them why, although it sounds like these pigs could use the feedback.
You do not have to rent to somebody you don't want to rent to. END OF STORY.
Actually, if they claim you did not rent to them for a reason like gender, national origin, or religion, or race, etc., and convince a judge or jury that's why you didn't rent to them - they can win a shitload of money from you in damages.
So, while I don't HAVE to rent to someone I don't want to - it might not be the end of the story.
For this particular property, it's a moot point. We decided to sell because the quality of folks in the neighborhood has been decreasing. It's fairly common to see bums with the aimless poor person shuffle wandering down the next street over. And since we don't live in the neighborhood anymore, the fact that we see a goodly number of them means the problem is most likely even worse.