First take a look at the user profile. If you see that all of the answers (or almost all of them) are self promoting, flag one of their answers for moderator's attention and fill in a message saying "This user only self promotes". You can even put the link to the user profile.
Most probably their accounts will be suspended temporarily or deleted.
No, they should not be allowed.
If the third party module developer is active there is a high chance he/she will answer your question if you contact him/her for support.
Allowing question as the one you described opens a dangerous door.
Someone may be active today but not next week.
In your particular case, I was the developer in question, I also closed ...
I think Magmi questions should be treated as off topic. If we allow one exception, soon we will have to allow all of them and this website will end up being a cemetery of un-handled extension support questions.
Each developer should support his/her own extensions.
If you have issues using an extension you should address it to the developer.
On the other hand ...
We don't start a subforum because we cannot. We are not in charge of creating websites on SE. And I doubt someone will agree in making such a sub-website because there will be a waste of time. here will be a lot of unanswered questions.
And extension developers should offer support for their work. If they don't offer support by email I doubt they will offer ...