I'm not sure that one can do what you need... as long as a user has the Assign permission he/she will be able to do it.
I haven't tried this, but you could try deleting the Assign permission to all users, and try to handle the assignee only from certain transitions in your workflow.
Give it a try... nothing to lose.
In the past, Portfolio for Jira required a high degree of detail–foresight that was unrealistic for many businesses to have–in order to produce a reliable long-term roadmap. We're tur...
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