As a Site Owner, you have the ability to choose which permissions are associated with a particular permission level and/or add new permission levels to combine different sets of permissions. 

I recently ran into an issue on our internal support site where contributors were accidentally deleting pages list, files and content from the sites. I wanted our writers / members to have permissions to view, create and modify as needed. But I wanted to prevent them from deleting any files or content. In order to accomplish this, I created a custom permission level and removed ‘delete’ as an option. Following are the steps to create a custom permission level and then apply it to a user/group for a site.
 
PART 1:  Create a custom permission level on your site

1. Click Site Actions, then Site Settings.
2. Under Users and Permissions, click Advanced Permissions.
3. Click Settings, and then click Permission levels.
   On the Permission Levels page you can create a new level (where you manually choose all the appropriate fields) OR you can copy from an existing permission level from which you add/remove the specific permissions you want to make available to your users.  The following steps show how to COPY from an existing permission level.
4. Click the link for the existing permission level from which you will copy.  (In this example I clicked Contribute).
   On the Edit Permission Level page for the group, scroll down to the bottom and click Copy Permission Level.
   The page title should now read “Copy Permission Level … “.
5. Enter the Name and Description you want to give your new permission level in the corresponding boxes. (For example, I entered Name = Contribute – No Delete  in the Name box and Can view, add and update but not delete in the Description box.)
6. In the Permissions section, review each of the checked boxes and select or deselect any of the check boxes as appropriate for your new permission level.  For example, I deselected the check box for Delete Versions - Delete past versions of a list item or document.
7. When you are done editing the new permission level, click Create.
8. The new permission level will display in the list.
9. NEXT… apply your new permission level to specific users/groups on your site.

thanks