Are you referring to Snapshot backups? OS X Lion- About Time Machine's "local snapshots" on portable Macs
If so, Time Machine is supposed to erase them automatically. However, you may need to update Snow Leopard to 10.6.8.
Open the Terminal in the Utilities folder and enter or paste the appropriate command line. Press RETURN and enter your admin password when prompted. It will not be echoed.
To turn them ON: sudo tmutil enablelocal
To turn them OFF: sudo tmutil disablelocal
Note that turning them OFF will also delete all existing snapshots.