I think there are basically two issues. Out and out completely off topic messages, which rally once answered once should not attract further replies is they just repeat the same thing, and subject drift which are a little more difficult.
One of the most annoying though is when somebody announces  something like I need nvda with X voic4e as it surely is the only voice for a screenreader. This is as the old saying goes often a red rag to a bull (OK Bulls are colour blind, so sue me!), and OK its true for that individual but not a lots of others.
I think in this case a simple link to the page telling folk of all the synths is probably all that is needed.
Brian
bglists@blueyonder.co.uk
Sent via blueyonder.
Please address personal E-mail to:-
briang1@blueyonder.co.uk, putting 'Brian Gaff'
in the display name field.
----- Original Message ----- From: "Brian K. Lingard" <bkl@ncf.ca>
To: <nvda@nvda.groups.io>
Sent: Tuesday, July 24, 2018 1:25 PM
Subject: [nvda] OT discussions

Dear Gene & List:
Yes, some off topic discussion is desirable, guess my primary beef is
subjects that do not highly describe the topic of discussion in the message
bodies.

I trust I am not coming across as moderator, however was commenting on list
posting habits generally.
Brian