restrict the user entries in the TMG - how to do

I have created table and table maintenance generator with the following fields:

MANDT MANDT

REC_INCREMENT CMS_DTE_SEQNO

WERKS WERKS_D

MTART MTART

MATKL MATKL

CREATED_BY SYUNAME

CHANGED_BY SYUNAME

My requirement is :

I need to restrict the user not to allow values in mtart and matkl fields with wild cards(*).

How to achieve this ?

YOUR HELP IS HIGHLY APPRECIATED.

Thanks in advance.