Validation Rule can be used to avoid update to Case record when the Status is Working.

Validation Rule Formula:

AND(
ISPICKVAL( Status, 'Working' ),
ISPICKVAL( PRIORVALUE( Status ), 'Working' ),
NOT( ISNEW() )
)

Note:

When the Case is updated with Working status, the Validation Rule will fire.

When the Case is created with Working status, the Validation Rule will not fire.

When the Case is updated from Working to a different status, the Validation Rule will not fire.