Postgresql 9.3 replication: Preventing wal-files from building up, if receiver goes offline
Set wal_keep_segments to a low enough value that WAL will be discarded before pg_xlog fills up.

If you're also using WAL archiving with an archive_command, Pg will keep on trying until it successfully sends the WAL or runs out of space. Don't have your master copy the WAL directly to the replica; instead put it on some reliable high availability storage that the replica also has access to.