Hi @Fursty - If I am understanding your question correctly, if the assignment was never published, you should not have to do anything; students would not see the assignment at all and it would not affect their grades in any way.
If this is a published assignment because you wanted them to see something (and even actually submit something if it is an online assignment), you could edit the assignment enable the checkbox to exclude the assignment from the final grade, as outlined in this online lesson: How do I exclude an assignment from the course's final grades? I imagine that will be the easiest solution for you in this case. I am fairly certain you can do this at absolutely any point in time; there is nothing within Canvas that would "prohibit" you from enabling this checkbox, even after you already typed in some grades.
And finally, just for your future reference, if there is ever a case where you want to excuse an assignment for an entire class--for whatever reason--you can also do this in the gradebook by recording a grade of EX. (Meaning "excused.") And no--you do not have to do this one-by-one. Canvas has a very nifty feature for giving every student a default grade for something. There are directions for this in two places, depending on whether or not you are using the New Gradebook:
I hope this helps, James!
