Unless changed from 13.1:
- Go to Admin\Objects and search for Idea Object - open it
- Click on Attributes Tab
- Navigate/scroll down to Status and click on it
- See "Read-Only" checkbox and mark it
- Click Save.
Alternatively, if you ever have a scenario where Status needs to be editable:
- start a workflow on Idea Create
- create an action step where Status is locked
- go to finish
On create, the process will run and lock Status, making it read-only and displaying a small 'lock' icon next to the field.
Of course, you'll need design an unlock workflow process in order to make the field editable, and define start conditions/access rights that fit your scenario. Perhaps manually started by Idea administrator, which when completed relocks the field.