Some customers will attract No excise
In that case, via CIN tab of customer master, this can very well be controlled. There should be some user discipline as far as SAP is concerned and you should not go for user exit to control such users' ignorance. Still if you want to try with exit, you can try with J_1I7_USEREXIT_EXCISE_BEF_SAVE but again, please discuss with clients' finance team and ask them to pass on the message that it is not a Best Practice but should be controlled by users' SAP knowledge.
G. Lakshmipathi