There's nothing in the core forum this, an add-on would be needed (I'm not aware of one with this functionality).
Rather than take this approach (which I presume is for newly registered spammers), you could prevent the Registered group from sending new conversations by revoking their user group permission. Then set up a user group promotion so that after a specified number of posts and/or time users are promoted to a secondary user group that does have permission to send new conversations.