directly from Oracle documentation :

Purging Rows from a Snapshot Log
Always try to keep a snapshot log as small as possible to minimize the database space that it uses. To remove rows from a snapshot log and make space for newer log records, you can:

Refresh the snapshots associated with the log so that Oracle can purge rows from the snapshot log.

Manually purge records in the log by deleting rows required only by the nth least recently refreshed snapshots.

To manually purge rows from a snapshot log, execute the PURGE_LOG stored procedure of the DBMS_SNAPSHOT package at the database that contains the log. For example, to purge entries from the snapshot log of the CUSTOMERS table that are necessary only for the least recently refreshed snapshot, execute the following procedure:

DBMS_SNAPSHOT.PURGE_LOG (
master => 'sales.customers',
num => 1,
flag => 'DELETE');


Additional Information: See the DBMS_SNAPSHOT. PURGE_LOG procedure in the Oracle8i Replication API Reference book for details.