You could modify it at screen level:
> SE80: Program SAPMV50A
> Display
> Screens: Screen 4001:
Element LV50C-DATBI / Type I/O: For this entry, deselect the "Input" flag.
This would ensure that the field is always greyed out.
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Also, the date is usually populated with the current date, but can also be populated with a different date if you have maintained user parameter LEDAT (See wiki User Parameters in LE-SHP - ERP SCM - SCN Wiki, number 2). So it could be a possibility that the user is not entering a future date but is coming from a user parameter.