DeviceN is the space for any single uncalibrated colour, usually a spot colour but could be process black. DeviceN is allowed in PDFX-1a so its not that unusual to find it. You could try setting Quark to output in the greyscale100K colour space, but I'm not sure that would prevent it. The main problem with DeviceN for pre-press is that if you try to use the Edit Image feature in Acrobat on an Acrobat DeviceN image in Photoshop, it can't open the file. Quick fix is to first convert the image colour space to cmyk using something like Pitstop before using the Edit Image feature, it then will open in Photoshop and can be turned to black only or greyscale and saved back into the pdf. You could also play around with Acrobat's Convert Colour feature.