Make Column Read Only Sharepoint 2010

PS C:\Users\gadekasa> $spWeb = Get-SPWeb http://abc:5552/sites/AirCraft/
PS C:\Users\gadekasa> $spList = $spWeb.Lists["AllWorkflowDefaultList"]
PS C:\Users\gadekasa> $fields =$spList.Fields["Role"]
PS C:\Users\gadekasa> $fields.ReadOnlyField = $false
PS C:\Users\gadekasa> $fields.update($true)
PS C:\Users\gadekasa> $spList.Update()
PS C:\Users\gadekasa> $spWeb.Dispose()