database-level locking

Does anyone know a way to lock a database under postgres while
leaving available the other databases managed by postmaster?

We are running postgreSQL 6.4.2 on our web site for content serving
and for registration information.  There are 4 separate databases
served by postmatser to support these functions.

Every now and the we have manually deleted temporary sort files to
keep disk usage in control.  We'd like to automate this process, but
to do so safely we need to ensure that no sorts are being run while we
delete the accumualted files.  Thus, we'd like to lock a database,
delete its accumulated wastage, unlock the database, and move on to
the next.

Of course the better solution would be if postgres was always able to
identify and clear out its excesses, but we have found that is not the
case.

Karl