Currently there is not a way to lock a company to a single database. In the past we stayed away from that because smaller environments like to start out that way with doing a database per company but as they grow it becomes harder to manage, especially in a DAG.

I do see your point about journaling but you can still journal at the group/user level but I know this adjusts the price of the SAL from Microsoft.

At this present time you would have to manually manage that on the backend. You can do this by setting the flag IsExcludedFromProvisioning to $true on the database and manually move all the users to that database.