Its a grey area...there are some lovely lochs up north where the owner kicks people off despite no protection order (I believe Tulla is an example). I heard of this happening on Ba the other day, when most people assume this is free fishing.
I think there is very little they could do legally....you could politely ask them to direct any civil action to your solicitors and carry on fishing
On the matter of a stank...basically this is a man made water...ie a dam. The fish in a stank are actually property of the owner and the offence you would be commiting is theft.