Let’s say you have people in your ‘Contribute’ permission level but you really don’t want them to delete any list items. Modifying that level is very easy to do.

  • Go to : Site Actions/Site Settings/Site Permissions
  • Click on ‘Permission Levels’ in the ‘Manage’ ribbon group
  • Click on ‘Contribute’
  • Uncheck ‘Delete Items’
  • Click the ‘Submit’ button

That’s all there is to it.