You need 2+ 2 cost or less blockers that can kill or neutralize the Slueths/blockade runners Really, anything with black guns will work, but there are several units that make the job much easier. You need multiple because of the likelyhood that if you only have one it will get Swindled.
The following units are uniquely suited to handling Sleuths and/or Blockade Runners:
-Heavy Stormtrooper Squad
-Advisor to the Emperor
-Emperor's Royal Guard
There are many other units which are good choices, but these stand out as big problems for Sleuths. I am a particular fan of the Trandoshans because they also come with Heat of Battle, which is another really good answer.
Realisitically, you want to get 2+ units out turn 1 and then stifle their first couple of attacks. 2 gun units or those with tactic icons that win the edge are well suited to this. If you can mitigate their damage, they won't hold up long.
Heat of Battle is also very nice since even 1-gun guys can kill them, and double heat can cripple your opponent, especially if you have a tactic to drop after as well.
The DS deck I ran at worlds was:
I did not have any trouble dealing with Unblockables all day =).
2014 Star Wars: The Card Game US Nationals Finalist
2013 Star Wars: The Card Game Worlds Top 16
8xX-wing, 5xB-Wing, 2xYT-1300, 4xY-Wing, 5xA-Wing, 2xHWK-290, 3xE-Wing, 7xZ-95, 2xYT-2400, 2xGR-75, 1xCR-90
10xTIE/LN, 11xTIE/IN, 2xTIE/Adv, 4xTIE/as, 3xTIE/Ph, 3xTIE/D, 3xFirespray-31, 3xLambda, 2xDecimator