How to ... implement characteristic validation forbidden infocube update?

Hi Gurus,

I need implement into SEM-BPS a validation so that the update of certain combination of characteristic be forbidden.

I build up and exit function.It gives an error message when this combination of characteristic be done, but then if the user push save button the information is update into the infocube.

Someone can help me? how to ...implement this validation forbidden the update into the infocube?

Thanks in advance for your help.