Information Rights Management;

office.microsoft.com/.../HA101280011033.aspx

It's the most secure solution  you're likely to find but it is not easy to implement!!

I advised a questioner on Experts Exchange about this a while back. Should you decide on the IRM route these selective excerpts from that thread might help;


"Before IRM can be enabled for lists and libraries, the Microsoft Windows Rights Management Services (RMS) Client with Service Pack 2 must be installed on every front-end Web server for your SharePoint site. In addition, Information Rights Management needs to be enabled in Central Administration for your site. Additionally, a server administrator must install protectors on all front-end Web servers for every file type that the people in your organization want to protect by using IRM."


First of all, installing anything like this you need to back up your environment. Not just the Content Database, but your whole server. This is something you should have available to you for when you install Service Packs etc.

You can then obtain the RMS client from here;
support.microsoft.com/default.aspx/kb/917275.

IRM Protectors, there's some info here which you need to read  carefully. Didn't find any info about pre-made Protectors I'm afraid;
msdn.microsoft.com/en-us/library/ms439253.aspx
msdn.microsoft.com/en-us/library/ms436515.aspx

To code Protectors you will need someone who understands C. You are not going to like what you see in this post ;

social.msdn.microsoft.com/.../
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