App Menu > Dupe Blocker > DupeBlocker Settings
By default you can use DupeBlocker for Account, Contact, Lead, Lead to Account and Lead to Contact. But if you want to use DupeBlocker for other Standard or Custom Objects you need to enable required objects. Select the object (For Example: Campaign) and click enable.
I have enabled DupeBlocker for Campaign object. You can remove the dupe blocker by clicking disable.