DISABLE SQLSERVER VSS backups
You will need to do a trace or Extended Event session to determine what application is attempting the backups. Be sure to include the client hostname and application name, those two generally help indicate what is taking the backup. The client hostname will most always show the local machine name, which if that is the case look at what is installed on the server.

Some examples of 3rd products that will use VSS:


Symantec NetBackup SQL Agent
Symantec Backup Exec ( I believe does this now as well)
VMWare/Hyper-V (both products have options to along with snapshot of server to also do database)