Well, if it's one object then you could change the drop action to simply "msg <You cann't drop #object#... Sorry>"
Or maybe you could make a type, with:
"msg <Sorry, but you can not drop $displayname(#quest.lastobject#)$ .> "
Or something like that.
There seems to be no information on standard messages in the help file at all - must be an oversight on Alex's part... I'll have to look into this...
EDIT::
Oh, it's right there... Says:
"player tries to drop an undroppable object"
You mean there... But you want a custom #@object# thingy eh!?
try either #@quest.lastobject# or $displayname(#quest.lastobject#)$ ... I would try but my QDK won't save (pffttt, I upgraded a few weeks ago - not a whole year ao

).
Ahhhh well, I didn't use QDK that often anyway - although it' a second reason to buy...