I've recommended this several times. Ideally that policy would also apply to profiles, since many of the spam events are limited to posting links in the profiles.
It looks like about 20 spam accounts are created per week, and only a few of them make posts. The rest are changing their profiles to include links and images. Forum moderators are deleting spam and banning spam accounts every day or every few days. It would be great to restrict new accounts by default, although then the problem would just be delayed since the bots would wait until their restrictions were lifted and start posting spam. The person to talk to about this is @implodedok There was a mention of a paid module for the forum that would help to prevent new spam accounts from being created. And, maybe switching to different forum software would provide more spam control features.
But, it's not a pain point for implodedok because he doesn't do moderation of the forum from what I can see. It's a pain point for the forum users and moderators like me. We keep squashing spam, but it's reactive instead of proactive. Google indexes it within hours, not days, and it affects our PageRank.