The red stamp didn't work by itself. It relied upon a process. In my experience of such systems (which was many years ago), when a document changed, the document control people would locate all the controlled copies and update them. The stamp was red because the photocopiers that were in general use at that time would not do colour. Training everyone in document control was simple: look for the red stamp and don't use anything that's got the stamp in black without first being darned sure it was up to date. In large organizations it only worked because the document control people were fearsome.
Whatever you write on the changed documents, electronic or paper, red stamp, watermark or message about being uncontrolled, it won't work without a process at the supplier to make sure people use correct copies. For example, writing "uncontrolled if printed" will not work if people at the supplier do not know what "uncontrolled" means, why it matters and from where to get the right version.
(It will be even harder if you're ordering x amount of product to design drawing version 2, and a smaller trial order of y amount of product to design drawing version 2.1 because in such a situation you expressly do not want everyone working to the same version: current production is at version 2, but trials of the next version are at 2.1 and your orders and quantities are different.)
It seems to me therefore that you should insist that your supplier has a process for managing document change, such that the right versions are in use. For your ISO 9001-certified suppliers this ought to be easy since they must have a process for controlling documents of external origin (including yours).
For non-certified suppliers, if it were me I would either require them to control documents I send them in an ISO 9001 fashion (and go audit to make sure), or ask them to get certified (because if they can't control docs, what else can't they do?).
If I and the supplier were truly in the 21st century, maybe, just maybe, I'd make the docs available on a password-protected website that I control with print privileges disabled (only works if they have internet-connected computers at all points of use and no need to print).
Hope this helps,
Pat