Assuming that this is a case of one user serially close voting/flagging the questions of another specific user, the main thing to do is to honestly review each question independently when you see them in the review queue, or otherwise come across these questions.
The system has checks and balances to prevent almost all malicious close voting/flagging. The most basic check is the requirement of multiple users to close a question (excepting when a moderator votes to close, or a gold tag-badge holder votes to close as a duplicate). This first check makes it generally unlikely that questions are closed without any justification. And close votes expire after four days if the question is not closed in the meantime, meaning that there is a limited window during which the malicious intent can amount to anything.
Even if closed, recently closed questions appear in the
/tools?tab=close page (for 10K users) for further scrutiny. And the community has developed methods (the C.R.U.D.E. chat room, as well as the Requests for Reopen & Undeletion Votes, etc. meta thread) to expedite the reopening of questions which should not be closed.
In all, I view "serial" close voting/flagging as a rather limited issue not generally requiring special reactions. Users simply reviewing these questions individually and honestly as they appear in the review queue will almost always be enough to thwart any purely malicious intent.