You need to lock down the permissions in the SQL database to prevent this sort of thing. It sounds to me like the security of your database has been compromised. You need to have a database administrator help you with this. You might also want to post your problem in the SQL forum instead of this forum. You should be able to turn auditing on at the database level. You'll also want to audit each and ever user account within the database to make sure that only current employees are still active in the database itself. If you don't have a process for deactivating users within the database when they leave the company, you've left yourself wide open to possible security issues right there. All it takes is one compromised user account within the SQL database itself for you to be in big trouble.
Good luck,