I think that there is only one way to fix it - it's to disable making
*hard*links to directory with mode 1777.
I don't use quotas, and don't know a great deal about how they operate, but I think there's another disk filling DOS involving hard links lurking which the above measure would also solve.
If a user starts making hard links to (large and growing) log files, with the new links being placed in /var/mail, then presumably those log files will not be deleted correctly as they are rolled over, and will quickly accumulate.
This could not bring down a system as rapidly as growing the publicly writable directory with lots of links, but it is not desirable system behaviour.