I have received many complaint about scam on my DM severally. Then warn people, pin relevant posts to educate them and drop announcements still, some people would still fall victim. Even with all these efforts, some users still trust random DMs more.

Could you make it so that people can't DM until they have a certain amount of posts on the forum?

Scammers are everywhere, especially on forums. What you need to do is educate your members about scams, do pin warnings, and monitor their DMs. Despite all of this, some people will still fall victim. So, you can't save everyone.

I pay close attention to who joins the forum.

I typically add custom user profile fields that ask the joiner why they’re joining. I manually approve everything. I research each person who joins unless I know who they are.

I find this is the best way to mitigate at least 99.999% of spammers.