In The Raiser's Edge 7.91 and below:

To remove constituents:
On the Criteria tab, add the following field: Constituent Information > Specific Record does not equal (or not one of) [specific records]

To add constituents: 
On the Criteria tab, add the following field: Constituent Information > Specific Record equals (or one of) [specific records]

In The Raiser's Edge 7.92 and above: 

NOTE: This functionality is available in Giving Score query and in Query List
  1. If the existing query is a Constituent, Individual, or Organization query, create a Query List from the query.
  2. Highlight the constituent(s) you wish to add or remove and click Remove from List (or Add to List) > Selected Constituents.
    NOTE: You are able to highlight one constituent, hold down the Shift key and then select another constituent, which will select all the constituents in between.
    OR
    Click Remove from List (or Add to List) > A Group of Constituents and search for a query of records to add/remove.
Query List screen shot