Hi Wim,
There's no way around this currently in Totara, without customising the code. We are scoping out a new Centralised Messaging Library with a view to enabling more effecting management of messaging site-wide & I've brought this thread to the attention of my colleague who is managing the project.
Unfortunately in the short term there's not a great deal we can do, the functionality you need isn't there, possibly the only effective workaround would be to use some kind of content filter in the email server.
regards,
George.