You can use a pair of macros to intercept the built-in commands for those buttons:
Sub FormatBulletDefault()
Selection.Range.Style = wdStyleListBullet 'or specify some other style that you want
End Sub
Sub FormatNumberDefault()
Selection.Range.Style = wdStyleListNumber 'or a different style
End Sub
Note that the above code won't help if users click the arrow on the button and specifically choose a list format.
Stefan Blom, Microsoft Word MVP