Ok, but then you'd need to explain better what you need. You are leaving user D out of the calculation. Is that what you want? If so, you just have to include another slicer for User and select users A, B, C only, leaving D out
Another option would be not to use this User slicer and embed the filtering in the measure
" You are leaving user D out of the calculation. Is that what you want?" - correct, but this should be done by calculation and not hard coding. Since user D doesn't have code "A1", so D have to be filtered out.