Thats pretty much the way to go.
The only other option would be to create a custom group with forum permissions off for topic creation and replies.
Then use the member management search and use the action at the bottom to move them all to the new group.
But you would also need to change the default user group for new registered users to the new group.
The only advantage of this is you could preserve the existing user group permissions that allows posting and then manually move members to the group again to allow posting on a user basis.
But you do the reverse and create a group that allows posting and manually move members to that group.
