I have a similar problem in my shop... When it's a real issue, I have to just create my own crop marks in the corners and set it for bleed. We don't do process work, so the pressman just uses the crop marks for registration; we don't need the bullets. I don't think there's any way to turn off the color bars. There is an XTension somewhere, I forgot the name, that can create custom color bars, so maybe that would help you avoid using Quark's generic Registration option.