Disallowing a process to be published when there are unassigned activities would stop that from occurring.
Either that or, as a minimum, a warning pop up when the publish button is clicked.
Alternately, make it a config item that clients can control eg 'Enable publish with unassigned activities' (default setting would be off)
This is not on our near term roadmap but we'll open this up for voting to gauge interest from other clients.