A recently published devWorks article details the Enhanced Quota Feature in Domino 8. This enhancement will proactively prevent the creation of memos, calendar entries, and to-dos if a mail file is at or near a certain quota threshold. In prior releases, the user would be able to begin the creation of the document and receive an error when attempting the save operation. This takes the quota enforcement a step further.
Note that the document states that this can be enforced via policies. I have not been able to find these options in any of the policy documents, though. The notes.ini settings can be pushed down via modifications to the policy documents.