I believe there was a trick to turn off element/module creation but I'm not recalling at the moment. I'm thinking, once the PI Units are "locked" the interface won't add any more. I don't have a batch test environment anymore so I can't "play" around .
Also, there might be a trick to use recipe templates and define a custom trigger. I'll think about this a little more and post back if I think of anything. Recipe templates are really powerful for defining true customization, but can take time to get right. Tricky questions coming from your company!
This is not currently possible, but I have run into similar scenarios before so I have created a Uservoice item to track this as a potential enhancement for a future release.
If this would satisfy your requirements, can you please add your vote and/or comment?
In the meantime, I believe I had some success in the past by modifying Module/Element-level security so that the interface does not have permission to create any new Units.