Using ACL to allow users to add files but not subdirectories in a folder
EDIT: The following is a partial solution. It will work at the shell level as expected but in Finder it can be bypassed with the if user is admin or knows the a admin user id &amp; password. See comments below... 

Check the man page on chmod and you will find...

 The following permissions are applicable to directories:
       list    List entries.
       search  Look up files by name.
       add_file
               Add a file.
       add_subdirectory
               Add a subdirectory.
       delete_child
               Delete a contained object.  See the file delete permission
               above.


So the command you are looking for is...

$ chmod +a "staff deny add_subdirectory" test


to forbid anyone in group staff from creating sub folder and then testing should give you 

$ mkdir test/subtest
mkdir: test/subtest: Permission denied


Where as creating a file...

$ touch test/blah.txt
$ 


is no problem. You should also find that Finder will have the Create Folder option disabled as well.

HTH