Remove all unused master slides in a PowerPoint presentation

You can do this by creating a macro:

Sub SlideMasterCleanup()

Dim i As Integer
Dim j As Integer
Dim oPres As Presentation
Set oPres = ActivePresentation
On Error Resume Next
With oPres
    For i = 1 To .Designs.Count
        For j = .Designs(i).SlideMaster.CustomLayouts.Count To 1 Step -1
            .Designs(i).SlideMaster.CustomLayouts(j).Delete
        Next
    Next i
End With

End Sub

View as "slide masters". When you mouse over each slide master, you can see if that slide master is used. You can individually delete each slide master. Looks especially for those with pictures and remove the ones not used.

If you highlight the first slide, then keep hitting Delete repeatedly, only the slide templates that are not in use will be deleted. This is an easy way to manually roll through the templates quickly without worrying about removing one that is in use.

The upvoted answer above still works and did work for me. The problem was that, when I copie the code I did not know what to do with it.

After a little research, I found that this is how to use the code above:

1. Open your PPT doc
2. Go to View, Click Macros
3. Choose where you want to be able to run this macro in the dropdown
4. Give it a name and create it.

Then a window pops up where you can paste the code.

Save it, go back to View-->Macros and Select the macro from the dropdown.

Click "Run" and DONE!