I think this is one of the main motivators to move from PDFx1a to PDFx4. Ofcourse if the image is a placed add that at some point in its "life" has been flattened there is little you can do, possibly, you can edit page, to move the image to Illustrator there select all the objects and Rasterise them, (or export to PSD) that would merge them, and then continue from there, but it is much more work to reconstruct the file that has images converted to tiles than aviod the tiles. I would probably contact the customer and tell them what image needs to be fixed, since the risk to just fix it would be too high.
What is the issue you are trying to fix btw? If it is ink limit, try using the colour converter in Acrobat pro 9