Here's how to disable Decnet V dns from an adapter:

from the DECnet-OSI V7.3-2 ECO 1 release notes:

The logical name DNS$ETHERNET_DEVICE can be used to provide a list of
devices that DNS should NOT use. All devices must be in the form _xxy0:.
The string can contain spaces and other text which is ignored by DNS.
For example, the following command tells DNS not to use the _EIA0: and
_FWA0: devices.

$ DEFINE/SYSTEM DNS$ETHERNET_DEVICE "Don't use _EIA0: and _FWA0: "

$ DEFINE/SYSTEM DNS$ETHERNET_DEVICE "Don't use _EID0:"