I would get out the red pen and for those that did the paper give them the grade they earned and continue on. For those that did not do the paper, give them the f (even if the work is shoddy those who did it deserve the counting of their efforts)
Man I have been there and don't envy you. I failed half a class of fifth graders once for just the same thing, half assed work and no work. To me it is unethical to compromise your curriculum and standards. Contemporary education has lost its balance between the needs of the student, the needs of society and the goal of the proffesion
There are still the sixty percent that did it the right way, so I would help out the clueless by flunking them and making them take it over so they can make it through the rest of their college years, and pray non of them are ed majors. It would be in the long run the best thing that happened to them, to have to shell out more money to take a 100 level course might just be the needed clue by four.

If you want to be a bastard pass all of them and let them flunk out of college when they hit the 300 and 400 level classes. These are fundimental skills for college citation, of sources, following directions and paying attention.
I hope this helps.