We never found an automated way via SMSQL to remove older reports, so instead we created a scheduled task which consists of the following command and put it on all of our SMSQL/SME machines:
Forfiles -p "C:\Program Files\NetApp\SnapManager for SQL Server\Report" -s -m *.* -d -30 -c "cmd /c del /q @path"
which removes all files under that path which are older than 30 days. We run it once a day and it prunes the files for us.
Hope this helps!
Jason