Howdy,
I had a look at the scripts that IBM provides to configure DEF which are found at:
<Root>/BPM/Lombardi/tools/def
We find scripts to create and reload the DEF configuration but no obvious scripts to remove/undo those steps. What follows are some educated guesses/thoughts.
It appears that the primary script for DEF configuration is is "SampleConfigureEventsToJMS.py". If we delve inside that we find a number of wasadmin commands that appear to create the DEF definitions.
Stepping back from this, in principle, there should be corresponding wasadmin commands to "undo" the create requests. However, it would take time for either me or you to try and decode those commands to build the correct sequence.
If we think about what wasadmin does, at the highest level, it modifies the WAS configuration settings which manifest themselves as XML files that exist on the file system.
If we look, we will find (for a standalone BPM server):
<Root>/profiles/<serverName>/config/cells/<CellName>/defconfig.xml
This file is likely the one generated by running the wasadmin commands.
If it were me, I'd take a sandbox BPM, shut it down, save that file somewhere safe for restoration (if needed) and then remove it and then restart BPM. My gut says that this would disable DEF from then on.
However, since I personally would consider no documentation to disable DEF an omission, I'd recommend opening a support ticket with IBM and ask for a formal resolution that they can then include within the manuals so that future users would have a better answer.