As an ex-city boy, I have seen and dealt with a lot of crazy people. If the guy is in the store acting nuts, but not threating or hurting anyone, legally you can't even pepper spray him. You tell him to leave, call the cops and take a defensive posture until he is gone. If he is saying threatening things I would still take a defensive posture, unless he approaches you or your family. Most of these guys are verbally ranting. I have seen homeless guys on street corners, scream and yell some foul and scary stuff at people as they walk by, but they never leave the spot they are standing on, they will never get in anyone face or grab them. If They do they are a threat, if they don't they are just ranting. Pepper spraying a crazy guy is BAD NEWS, whatever drug, booze or mental illness is making them crazy, might also lessen the effect of pepper spray, and make him more angry/violent. I would only use it as a last resort. There is a video on the internet of a bunch of cops pepper spraying a Wacko with no effect to the guy. Here is a link, but be careful of the spyware.
http://www.m90.org/index.php?id=9297
http://www.m90.org/index.php?id=9297