If there is a way to do that, I haven't found it for the Forum.
I'm not an expert on SMF but I have found a reference to version 2.0 in relation to combating spam attacks that says under Security and Moderation features: "you can even require verification until a member reaches a certain amount of posts." That's what I had in mind.
I'm aware that some human spammers make essentially meaningless replies in threads to build their post count, but if the mods scan what the new members who are 'on hold' are writing then it ought to be obvious that their posts are bogus.