@bluefinstudios it is not possible to prevent people from withdrawing any of their own stake, no. Something like that would require a hard fork, and, that would likely be a very controversial topic - even if it had 'good intentions'.

However, if you notice people abusing their free 15SP delegation loan, you can submit here for it to be removed: https://github.com/steemit/redeemer-irredeemables (this has been done for thousands of accounts)

The way the free delegation loan currently works is that their delegation is slowly removed as they gain more of their own SP. It's also highly reduced if they are inactive for a long time (leaving just enough to become active again in order to keep good UX for returning users).