SPANK is an illegal drug that originates in the South American country of Colombia. The Colombian Cartel has the monopoly on producing it, although other gangs can distribute it in Liberty City. This drug is mentioned several times in the video game Grand Theft Auto III. The side effects turn people into crazed zombies who would kill for another taste of that drug. The overall objective is to make sure that SPANK is either eradicated or strongly contained in the confines of Liberty City.
SPANK is obviously a pun on "smack", once more proving Rockstar's intent on applying a good sense of humour to their games.