De or het banvonnis?

The correct Dutch article is:

het banvonnis

Exception Words ending in -nis usually take de, but banvonnis is an exception. See the rule

How to use banvonnis

the banvonnishet banvonnis
this banvonnisdit banvonnis
that banvonnisdat banvonnis
our banvonnisons banvonnis
your banvonnisjouw banvonnis
every banvonniselk banvonnis
which banvonnis?welk banvonnis?
a big banvonnis / the big banvonniseen groot banvonnis, het grote banvonnis