>You could just create a custom filter using the predicate function. Check if the operator is included in the 'already selected' array and if so return false, otherwise true....

I am not sure I understand you exactly, but my colleague made it to work from his directive (that was the main challenge, as we do already have a filter that works correctly of filtering items from the array). He eventually didn't use the filter assignment directly, but did something different. Anyway, it seems to work fine.