I figured out a workable solution. Since the domains I want to do this for only have a couple of addresses, I was able to configure a RegEx to block all mail going to addresses that started with something other than the first letter of the legitimate email accounts. In the example scenario above, all the addresses start with a 't', so I would tell spamfilter to reject messages that have to addresses that match this:

(^[abcdefghijklmnopqrsuvwxyz].*@test.com)

Explanation: the () are necessary for the black list to interpret it as a RegEx

the ^ designates look at the beginning of the line

the [abc..] (notice the 't' is not there) matches one and only one of any of those characters

the .* matches anything after the first character, regardless of character or length

the @test.com matches "@test.com" (actually it would also match @testacom, @testbcom, @testccom, etc, but that's not really a concern. If I only wanted it to match "@test.com" I would put in @test\.com so the Spam Filter would know that the . is actually a period and not a placeholder for any character.)