To change the status of the number / agent to inactive, you can click on the green status pill 'Active' found next to the profile picture of the number / agent in question.
Clicking this will set the number / agent to Inactive.
You can simply click this again to reactivate it again.
Alternatively you can also change the status of the number / agent via the edit window, accessible via the settings cog icon at the end of each number / agent row.