You can get your logic to work both in a Load Script and also in the front-end.
In the load script you could use Not( Exists( 'CompositeKeyField' , CompositeKeyValue ) ) in a WHERE clause of a LOAD statement.
Field In ('a','b','c') can be written as Pick( Match( Field , 'a','b','c' ) ) in QlikView.
In the front-end you could query it by at least two approaches:
1) Just by using the associative logic of QlikView - under the assumption that you have tableB loaded an associated correctly with the fields you want results from. Then you do the "Select excluded" option on a List Box after having selected the values.
2) By using Set Expression in your expression(s) in a chart.