We have a range of groups covering teams/departments, projects, action groups, work groups, social, etc. and in most instance posts are created in the correct/best location. We don't restrict posting to All Company, however we have in place guidelines about posting what and to where, which is covered in staff induction (Yammer self paced online learning undertaken in a private Yammer training group)
Below is wording which is in the Location, Location, Location document (referenced in the Yammer training), which may be useful
Q: Where should I post content?
A: The idea is simple - consider who you want to read your content and then post the content in the group that most closely matches your audience.
If your audience (who you want to reach) matches the members of a group and the purpose/topic of the group, post to that group
If there are a few people who might have 'extra' need to see an item who aren't in the group you are posting to use @ symbol to notify those individuals
If have a wide audience you want to alert, post to the most appropriate group and then share to All Company
For casual or non-business posts, always use a social group
Avoid posting to the "All Company" group unless it truly warrants whole of company notification
Remember If a post is created in the 'not best location' it can easily be moved using the move option (see https://support.office.com/en-us/article/Move-conversations-in-Yammer-149c6399-4ac1-4ced-84d7-e0660960a872). If you have a moderator they can recommend to the post owner to move the post.
Also consider that many people post from the publisher in their Home Feed, and unless they select a group to post to, the post goes to All Company.. Awareness and training helps to change behaviour.
Hope the above is useful.