Pro Sites Quota Limit For Free Users

Pro Sites has a built-in, "Free" level by default.

Using Pro Site's quotas, you can place limits on the number of posts members can make.

You can place different limits for different Pro Site Levels, however, you can not place limits on the built-in Free level account.

How can I limit the Free level? I want to allow the Free member to post no more than 3 posts.

I found a similar request from 2 years ago: