As per my knowledge there's no way to "ensure data are secure". You could allow dev to only access views instead of tables and use "TOP x" inthe view definition. But then you'll run into the risk of wrong results.

Another way would be to prevent access from a removeable device such as USB stick or any CD writing device together with a strong monitoring of outgoing mails.

But this would make it only harder to steel data, not impossible.

There's only a single method I know of: trust. If there's any lost of trust, access to sensitive data should be removed immediately. But even then it might be too late. You'll never know (unless you run a permanent profiler trace and analyze the captured data.)