The "Additional home directories matching the following value will also be used for new home directory creations." setting in WHM under "Basic WebHost Manager® Setup" allows the home directory to match a particular prefix so mounts with that prefix can be used for account creations. In setting this up at some point, you may want to force new accounts to be created to a particular home directory.
Access WHM and go to "Basic WebHost Manager® Setup" and set "Home Directory Prefix" to the path desired. In this case, let's force /home by setting this to default.
Next, select the custom radio button for "Additional home directories matching the following value will also be used for new home directory creations." and clear the text box so there is no value and then save.
Now new accounts will be forced to be created in /home even if/home2 exists because there is now a value set to match other home directories for use.