We have completely switched to InDesign so I don’t have Quark anymore. I did do a lot of Quark scripting though.
Try opening a page with items on and off the page and then get properties of each one and try to identify what is different about them and what property might identify it as being off the page. There might be a property that directly tells you or you might be able to check if the x and y coordinates are larger then the page size.
Anyway, select a box on the page and run this:
tell application "QuarkXPress"
get properties of selection
end tell
Model: iMacintel
AppleScript: xCode 3.0
Browser: Safari 525.20.1
Operating System: Mac OS X (10.5)
-- This works for a single page document
tell application "QuarkXPress"
tell document 1
set selected of every generic box of spread 1 to true
set selected of every generic box of page 1 to false
delete (every generic box whose selected is true)
end tell
end tell