SPList.WriteSecurity – make a SharePoint list kind of read only

According to the description of MSDN you prevent users from editing items of list using the property „WriteSecurity“. For this you need to change the value to „4“. You can do this via GUI or using the PowerShell.

PowerShell

$site = Get-SPSite http://sharepoint/sites/site1
$web = $site.OpenWeb()
$web.AllowUnsafeUpdate = $true
$list = $web.Lists[„ListToProtect“]
$list.WriteSecurity = 4
$list.Update()

It is important to know that this property only affects users which are members of the site collection administrators.

Resources:
SPList.WriteSecurity property (Microsoft.SharePoint)