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First and foremost, an InedoAgent will respond to any request with the proper AES encryption token (like a password), so if you don't want BuildMaster talking to a server/agent, then don't configure a valid connection to that agent.
Are you connecting your Standbox server to the same SQL SErver database? This is a big problem, don't do it...
** Never have two instances of buildMaster talk to the same database **
Are you restoring a database back up to sandbox? It's strongly not recommended, because of the problem you describe. The agents will be configured to talk to eachother.
Instead,, when setting up a sandbox, create a representative and testable version of your applications and processes. /you'll never be able to get a "perfect test" (since you can't talk to exact same servers), so at best you should test the broad functionality instead of the connectivity.