So, there was an Idea exchange entry for this that I’ve tried to comment on multiple times and it keeps getting lost in the ether somewhere… As frustrating as that is, I figure I should be able to at least post something here. I’ll go ahead an plug the original Idea Exchange post as well:
Even if the need is a little niche, I agree that this would be useful functionality to have baked into Polaris. In the meantime, I wanted to share how staff can achieve essentially the same outcome with current tools… and by current tools I mean SQL 
This isn’t complicated, but you do need to be have permission to run SQL searches and create new record sets from their results… but if you’re already trying to work on some sort of Record Set exclusion, you probably already have those permissions. Anyway, you need to start by creating your initial Record Set (or opening one you already have) and jot down the Record Set ID in the top right hand corner:
Once you’ve got that, create/open another Record Set of Items/Bibs you want to exclude (note: your original Record Set and exclusion Record Set must be the same type!) and get the Record Set ID from that one. Now all we need to do is make a SQL query to get all of the Items/Bibs from the original Record Set that are not in the exclusion Record Set. If you’re working with Item Record Sets, it should look something like this:
SELECT [ItemRecordID]
FROM [Polaris].[Polaris].[ItemRecordSets]
WHERE [RecordSetID] = --Starting RecordSetID
AND [ItemRecordID] NOT IN (
SELECT [ItemRecordID]
FROM [Polaris].[Polaris].[ItemRecordSets]
WHERE [RecordSetID] = --RecordSetID of Items to exclude
)
If you’re working with Bibliographic Record Sets, your query will look like this:
SELECT [BibliographicRecordID]
FROM [Polaris].[Polaris].[BibRecordSets]
WHERE [RecordSetID] = --Starting RecordSetID
AND [BibliographicRecordID] NOT IN (
SELECT [BibliographicRecordID]
FROM [Polaris].[Polaris].[BibRecordSets]
WHERE [RecordSetID] = --RecordSetID of Items to exclude
)
Really simple queries that almost anyone can follow, even without markup 
Anyway, plug it into your SQL Search, create a new Record Set based on the results and voila! You have successfully excluded Items/Bibs on one Record Set from another!
Again, I think this should be implemented in a less convoluted way, so please visit/vote on the existing Idea Exchange entry. I just wanted to offer an alternative for folks frustrated by their current workflows. If you have other thoughts, questions, or notice something incorrect, please let me know!