MInimum grade

A student in England answered the question prompt of "Describe the room you are sitting in" with: F--- off. The student earned a score of 2 out of 27. In previous years, I might write about the obscenity as an answer, but after hearing that phrase a miniumum of three times a day this year, I've become immune to it.
Instead, let's consider the giving of credit for assignments completed. If a student turns in a writing assignment that answers the prompt either indirectly or not at all, and if the writing style lacks purpose, or if the grammatical structures are missing, what type of score should the student receive?
Lately, I've given a minimum score of 50% if the assignment is turned in. I suppose the better solution is to hand it back to the student and request that they redo the assignment.