You can do it by User Group permission level:
- ACP > Apps > Forum
- Click on the desired forum you want
- In the Permissions tab, type the User Group, i.e. Users
- You need to check two permissions:
- Can create threads without approval: DENY
- Can reply to threads without approval: DENY or you can GRANT this since this is a reply
Now users in the Users Usergroup will be moderated before their threads/posts will appear in that forum.