We need to give permission user to just contribute in list item i.e user justupdate status of his/her task but prevent to delete task from task list.How we can stop a user to delete item.
Go to the Site Settings of the root of your Site Collection. ClickAdvanced Permissions. From the Settings drop down click PermissionLevels. Click Add a permission Level. Create a new permission levelfor your users that includes whatever they need, just don't give them"Delete Items." You can look at one of the existing ones for guidance.If you're the command line type you can use "stsadm -oaddpermissionpolicy" to create the new permission level and "stsadm.exe-o managepermissionpolicylevel" to modify the permissions that levelhas. I think Gary Lapointe has some operations that deal with them athttp://stsadm.blogspot.com.
You need to use an Event Handler to trap this action
use events..........................