You can't. The fields are text, if someone puts something sensitive in them, that's up to the user.
The only thing you might be able to do is write or find a validator which can scan the text and return a failure message if they enter something you don't wnt them to. Of course, what they might enter is so variable, I don't think you really can do this, but that is your only real option.
One other option is to use our recently released PII Protector for JIRA add-on. It monitors sensitive PII like credit card numbers, social security numbers, addresses, etc. stored in Atlassian JIRA, reports it, provides admins with a convenient UI to manage it, to audit access to it, and optionally to hide or to erase it.
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