Best way to limit database sizes

Hi,

        I have a single Postgres server which will be hosting multiple databases belonging to different users. What would be the recommendation to limit the size of each of the databases?

I've considered creating a tablespace in a directory owned by the user , so I can use Linux quotas to prevent higher disk usage , but this turned out be a bad thought, as all the files are anyway owned by the postgres user and so disk quotas won't have any effect.

Now I'm contemplating having a cron job which checks the size of each tablespace belonging to a user and somehow informing the admin about db's getting over the threshold size.

Is there anyone who has tried a better  way to limit the db sizes ?

cheers

/Dev