Ok, I think the solution should be :
Yes , we can purge the database from SEPM.
Symantec Endpoint Security Manager: How is Database Maintenance scheduled?
http://www.symantec.com/business/support/index?page=content&id=TECH95880&locale=en_US
But :
My groups are synchronised with the AD. So I don't manually delete the clients.
Consequence, when a computer is deleted in the AD, and SEPM synchronize with it, it don't put in the database SEM_AGENT.DELETED to 1.
So to clean the database, is the best way to set DELETED from SEM_AGENT's table to 1 ?
Regards,
JL