Well use the fact that "STATSPACK uses foreign-key referential integrity constraints with the ON CASCADE DELETE option".

http://www.dba-oracle.com/tips_oracl...ge_utility.htm

Code:
SQL> select SNAP_ID,to_char(SNAP_TIME,'dd-mon-yyyy hh24:mi') from stats$snapshot;

   SNAP_ID TO_CHAR(SNAP_TIME
---------- -----------------
         1 07-mar-2007 11:35
         2 07-mar-2007 11:36
         3 07-mar-2007 11:36

3 rows selected.

SQL> delete from stats$snapshot where SNAP_TIME < sysdate - (1/(24*60)) ;

3 rows deleted.
Extend this to 12 months and test it. Good luck.