This has been a concern for quite some time now. Do some research online and look up Roger Nichols copyright concerns for Mix engineers.
Without getting into all of that, simply do this:
Bounce down any processing you use....outboard, plugin, etc...
Simplifiy the mixes to stems, but leave them (client) the raw tracks at the top or bottom of the edit page. That way they can recover your mix if they need to, but can get all freaky with the original tracks.
My method with labels is to print my outboard onto new tracks and bounce certain "secret" methods I may have for processing. I generally leave all of my crazy routing in place and just assign my bounced tracks to the same routing. I keep a "current" session on my backup drives, and give the label / client the updated session. Everyone is happy.
However, this still does not get you away from the copyright concerns....which is a bummer...but hey, sometimes a man's just gotta work.
On a side note. Anyone who is willing to try and do what you do, steal your methods, etc.....isn't interested in working with you long term anyway. Just lick your wounds and move on to another client. Eventually you'll have a plethora of clients that come back to you, love what you do, and are willing to pay for it. YMMV
