There isn't. Your only option is to hide the mailbox size field from the user setting page. To do this just comment out the mailbox size code in  C:\Program Files(x86)\SmarterTools\SmarterMail\MRS\Interface\app\settings\domain-settings\users\user.html. 

You then have to rely on the API to set the mailbox size different from your default.

You may be able to set up an API call that checks and sets the mailbox size limit periodically. That wouldn't prevent domain admins updating the account size, but it could be quickly returned to a valid setting based on how you set up the call. 

Will this ever be an option in the interface?

Thanks,