Hi Chang
The only options are to define that as a Sensitive Field Or use a User Exit to validate the data
For customer master checks before saving.
FM EXIT_SAPMF02D_001/include ZXF04U01. Enhancement name is SAPMF02D.
For Vendor master checks before saving.
FM EXIT_SAPMF02K_001/include ZXF05U01. Enhncement name is SAPMF02K.
Ajay M