Limit database size

In my opinion, the best option is this:
-every user gets a seperate db
-check the size of the databases in e.g. every 5 minutes.
-if the size of a db is larger then the user quota then take away the user's INSERT and CREATE permission. And store somewhere that this happened, then notify the user.
-If at the next check, the db size is under the quota, then give back the permissions.