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?
There is also a non-hacky way where the field is made mandatory. That will not take away the "None" option but you will be forced to select another one (unless you gave another value as default).
If you must get rid of None, follow Rambanam's advise.
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