What page?

Here is the issue I am struggling with today...where should I draw the line with late/no work? I have two or three students who ALWAYS hand in their work late or not at all. Usually when we do things like worksheets in the classroom I give them back to the students for corrections. Often I get a confused look and, "What spelling sheet?" "huh? What math page?" I understand that this problem is not one unique to my classroom. It will continue to happen throughout adult life for some of these children. However, I am wondering for the pace of my class and for my own personal sanity, where do I draw the line? If I just make them not do the work (worksheet grades are not usually kept) they will get out free-I don't have to worry about it either. Win for both of us right? Not exactly. Because then when we move on to the next topic and they have not completed the practice for the previous topic they are not ready and subsequently fall farther behind. However, when I hand the sheet back for corrections and recieve it back 5+ times with the answers still not correct, is it even doing them any good to do it a 6, 7, or 8th time? Where does fixing/finishing work stop being helpful? Is there even a solution??