Can we block users from putting “Unassigned” in the assignee field?

[GBE]

As Default, when a ticket is created, this is Unassigned and when we reviewed the ticket, we assigned a member of the team. 
When someone is assigned on the ticket, I don't want that he be able to Unassigned the ticket. 

Is there a way to block people to putting "Unassigned" in assignee field ? 

[Evgenii]

Hi, @GBE 

There is one more way, but it's kinda workaround ) It won't prohibit changes, but it will deal with consequences.
You can set automation rule, which will trigger on changing of assignee, and if field is empty - then set previous assignee.
Or assign manager and add comment, that "This bad person removed himself from assignee" :)

[Sravan Indukur]

You can manage this through the Permissions settings.

Navigate to Project Settings > Permissions and look for the Assign Issues permission. This permission is required for a user to assign tickets to others (including assigning them as unassigned).

To control this, grant the permission to administrators and remove it from project roles that should not have this ability.

[Sravan Indukur]

Refer the below link: 

https://support.atlassian.com/jira-cloud-administration/docs/manage-project-permissions/

[GBE]

If we do like this, it means that people can't assigned other people on the ticket. for a code review, for example right ? 

[Sravan Indukur]

yes