Two other solutions work on large amounts of copy:
1) Edit or create a duplicate H&J setting with less word space and assign it to the paragraphs in question. I usually find the defaults a little loose for book work and quite a bit too loose for ad work
2) With Quark's "Type Tricks" extension installed, select a range of copy and use (Mac keys, Windows may differ) control-shift-command with left bracke to tighten word spaces in the selection by -10 units. You can add the option key to tighten by only -1 unit. Using the right bracket with the appropriate modifier keys increases word spacing by a corresponding amount.
Hope this helps
George