Most of the guys I have seen violating it are around the Train area, so they might of just got back from the water park and really I have seen about 4 total. Mostly its the women that are violating it. As for it being posted I am pretty sure it is on all of the Rider Safety Guide signs at the entrance of every ride. And I think they all say shirt and shoes required to ride.
Also I found this from: http://www.visitkingsisland.com/public/visit/faq.cfm
"Is there a dress code at Kings Island?
Shirts, shorts/pants and shoes must be worn at all times. Proper swimming attire for Boomerang Bay is defined as one or two-piece swim suits without abrasive buttons, rivets, zippers or snaps."
Just as an example, I know that I have seen people have to put their shoes back on to ride Diamondback and White Water Canyon. Now on WindSeeker, Drop Tower, and my guess Invertigo you can't have shoes required due to it being a safety issue.