Hi,
I cannot remember the details, but basically you should be able to "validate" the leave request data with some of the methods of the BADI (actually isn't there two BADIs?). By validate I mean that you can make additional checks for the data entered in the leave request, and if there is something wrong, you can display an error message. Now I am not absolutely sure about this, but there might be a field NEXT_PROCESSOR (or agent or something like that) in some of the parameters of the method. Now you could perhaps check if that field is initial, and if it is, then just give an error. I am not sure if it will work like this, but this is what I would check first, and how I would most probably try to solve the problem.
You the BADI doesn't work, then you can of course always try to find some nice spot in the standard code for a small enhancement. If you are using the "new" leave request applications made with WDA, you have even more possibilities. Of course this will require some above-beginner-level skills.
As a general comment I would say that me as a user would be very frustrated to get an error message when submitting a leave request. I would hope to be still able to submit the leave request and then someone responsible for maintaining the organisation would get notified automatically.
Regards,
Karri