Create but no modify or delete

I'm trying to set the permissions on a folder to allow
users (through an application) to create files in the
folder, but not allow them to modify or delete the files
after they've been created. However, the file "create"
function seems to essentially be a combination of a create
(of a zero length file) followed by a modify (to populate
the file), which leaves me with the zero length file and
possibly an error message. The application creates an
Excel formatted file (.XLS), but I see the same behaviour
with the Excel application as well. Does anybody know if
such a configuration (create, but no modify or delete) can
be set up and if so how? Or am I out of luck? Many
thanks for any help.

Marc