Would it be sufficient to remove the 'New Topic' button from the forum front page?

No.

I suspect not, because users who know how MLF works could enter the correct URL to add a new topic.

Yes, this is the problem.

Is there a way around this?

IMHO: Not without a bunch of code changes. (but I'm not the programmer)

Any ideas, please let me know

If I interpret your wishes correct, a blog system will fitting your needs.

Tschö, Auge