How can I protect the "Created by" field from changing?

[Olga_Popova]

On the create/view screen of task is an information who created a task, but everybody can change it. How can I protect the "Created by" field from changing?

[Tarun Sapra]

Is this a custom field? As JIRA doesn't provide a system field "created by" system field. If its a custom field then you can remove the field from the "edit" screen in order to make sure it's only filled at the time of creation. 

If you are talking about "reporter" field then you can follow the approach as suggested by @Udo Brand

[Udo Brand]

Don't grant the Modify Reporter permission to anyone. Then you won't be able to create issues on behalf of someone else.