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.
Hey Community mates! Claire here from the Software Product Marketing team. We all know software development changes rapidly, and it's often tough to keep up. But from our research, we've found the h...
Connect with like-minded Atlassian users at free events near you!Find a group
Connect with like-minded Atlassian users at free events near you!
Unfortunately there are no AUG chapters near you at the moment.Start an AUG
You're one step closer to meeting fellow Atlassian users at your local meet up. Learn more about AUGs