you make field mandatory on field configuration then "None" option will disapper.
another way is you can modify velocity files for this check the following document
I have the same problem in my JIRA 5.1 instance. I changed one of my select fields to Required in the field configuration. Now it will not let me select None, which is an improvement, but it still shows None as an option, which is a problem still. Is that expected when making the field required?
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