Indeed that does seem incredibly difficult, my imagination goes to a cloud of poly-bots, perhaps powered by surface-based laser(s) and solar, such that they could be made to gradually approach the target orbit, attach to the object, and any that attach could help the others by accelerating the debris towards them.. then hopefully once enough were on, you'd have enough control on the object to de-orbit it? (Total space/engineering layperson here, just thought it might be a way around some of the tricky parts?)