The SE engine has safeguards that will spot excessive voting like this and revert them; a few votes, though, cannot be distinguished from regular critique (although the user is always encouraged to leave reasons when down-voting).
If you feel there is established vote abuse, it can be checked and action taken, but please don't do this just because one or two posts are down-voted. Ultimately, the votes are weighted, and one or two up-votes outweigh (in net rep) a good many down-votes.
Flagging one of the affected posts for moderator attention would suffice if it does warrant a peek.