Content Type Change Control
You can prevent users from making changes to specific content types in two ways, by using read-only content types and sealed content types. Each method has its advantages.
However, you cannot prevent users from creating content types based on a specific site content type.
Read-only Content Types
You can define the content type as read-only. Marking a content type as read-only warns users that they should not make changes to the content type. The user must then explicitly set the read-only property of the content type to false to make changes to the content type.
You should know, however, that defining a content type as read-only has ramifications when you make changes to its parent content type and attempt to push down those changes. For more information, see Updating Child Content Types.
Sealed Content Types
For more stringent control of the content type, you can define the content type as sealed. You cannot change sealed content types through the Windows SharePoint Services user interface, and you must be a site collection administrator to unseal the content type using the object model.
Sealed content types are not updated through push-down operations.
Use the Sealed method of the SPContentType class to get and set whether a content type is sealed. To set this property, you must have site collection administrator rights.