Restricting schema sizes

Hi,

What is the best way to restrict/limit the size that a schema can grow too in Postgresql?

If I want to have a scratchpad area within my database to allow users to create tables,
how can I control the size of the objects they can create?

Would I have to create a dedicated filesystem and then create a tablespace on that
filesystem. Users could then create objects in that tablespace until they fill the filesystem.

Is there a setting in Postgresql whereby I can allocate a quota of space within a
tablespace for each schema?

The other option I can think of is writing a script that monitors the size of the objects
within a schema. The danger here is that a user could potentially create a huge table
as a result of a bad query (cartesian join etc) and fill the application tablespace / filesystem.

Many thanks.