Print purge sheets to clear the stained ink line. Wait and do it again since the new ink will dissolve more as it sits over time.
My purge sheet instructions, from memory. I'm sure they are documented elsewhere, but I couldn't find them when I searched.
Print the QTR ink calibration separation file (inkseparation8.tif) in calibration mode. That will give you a clean step wedge of each color. From there you will know which channels need to be cleaned.
Assuming it's the Magenta channel (#3) for this example. Open the inkseparation8.tif in photoshop. Do not color manage it. Using the eye dropped select the 100% color from the #3 channel. Draw a rectangle over the whole page and make sure it's filled with the sampled color. Save that to a new file as purge_m3.tif, again not color managing the document.
Print the purge_m3.tif file using calibration mode of QTR. I found it only takes a few 13x19 sheets to purge a channel on the 3880. I used enhanced matte. if you use plain paper you may want to limit the ink and print more sheets. I purged the yellow and one of the magenta channels twice with a day in between. Then it was clean.