Hi @ChriChri,
I found a discussion on a setting to disable DMs https://github.com/tootsuite/mastodon/issues/6945. From that ticket it seems that it is not yet possible to deactivate DMs. Maybe it would also be a good idea to link both tickets.
Assumptions and ideas
If you want to implement this yourself, you might search the purism repo for how they surpressed the DMs to implement e.g. an auto-respond if you get a DM while DMs are de-activated.
I assume that the librem one version “not complies with the protocol” and ignores DMs sent while the sender is either ACKed or not expecting an ACK (don’t know the DM procotol of Mastodon)
You might discuss in the github repo how this should be best implemented.