Although you can use Altiris without SQL knowledge I would highly recommend learning SQL. In the example below I define how to use a spreadsheet. You can use the same concept using a SQL statement querying all the users excluding the ones you flagged for deletion.

Today, how are your users being populated into Altiris? If they are coming from another database depending on how the results are defined we could automate the process.

Another thought, is if you have Symantec workflow we could change the entire game. This could handle the entire HR employee termination process. 

1. User flagged as Terminated.
2. Store custom data ( if required )
3. Assets identified
4. Confirm asset collection.
5. Disabled AD account
6. Either flag in Altiris or delete resource.

Really it boils down to what your company decides.

Basic Steps to remove users

1. Create a spreadsheet of all the Users you want to remain in in Altiris.
2. Under connectors create a new data source that links to the spreadsheet.
3. Create a new import rule that will review the users in the spreadsheet and remove the missing items.

 Please test the rule before running it.