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.