Reviewing through your description, there is not currently an option to set a default post place on a per user basis. However, there are a couple of options that might work for your use case.
1. If users are creating content from within a specific place, that place should be selected as the default. For example, if I select the option to create content here in the Jive Training and Support Resources space:
2. You can also create a direct link for these user to select when posting content which will set the place for them automatically. This can be done by navigating to the place you are wanting to create content in, selecting the content type you are wanting to create, then copying the URL found in your browser and providing this out to your users. Using the Jive Training and Support Resources space as an example again, to create a document, the link would look like - https://community.jivesoftware.com/document/create.jspa?containerType=14&containerID=2096