How to reset a postgres database that has used all of the available disk space?
First I'd have a look around /var/log to see if there are any files I could get rid of. If that's the case, you could then start your service and remove the appropriate databases.

If you do not care of your data, you could drop your cluster with the pg_dropcluster command. Look at the man file for the appropriate options.

And by the way, 8.4 support has ended a while ago (https://www.postgresql.org/support/versioning/) and you shoudl really consider upgrading to a supported version.