Is it possible for the people of a democracy to overrule the government's decisions?

The answer to that question depends upon what kind of decision is being made.  People have some recourse under some circumstances to seek an overruling of a government decision.  However, there are some government decisions that could not be overruled without voting those who made the decision out of office, recalling them, or through a revolution.  Let's go over the kinds of decisions governments make, so we can look at some ways they can be overruled.
First, enacting statutes is a form of government decision-making. In the federal government, it takes the Senate and the House to pass a bill.  There is no way to reverse this decision without a veto by the president, and even then, if there is a sufficient majority, the veto can be overridden by Congress.  This kind of decision can be fought against only by getting in touch with Congressional representatives and senators and asking that they revoke the statute just passed, by seeking a recall, or by not voting those representatives in again. This is true for declarations of war and treaties, too.  It is difficult, honestly, to imagine a revolution under any of these circumstances.
Second, the decisions of administrative tribunals and courts are another form of government-decision-making.  In either case, an appellate procedure is available to people, which can result in the overruling of the previous government decision.  Once a case reaches the United States Supreme court, though, its determination cannot be overruled except in circumstances in which Congress rewrites a statute in a way that will pass muster under the Supreme Court's ruling.
Third, the government can act in ways that can be overruled by invoking the protections of the Constitution.  For example, if I owned a newspaper and the government decided to order me to not publish some particular news, my recourse to overrule this determination would be to invoke my First Amendment right to freedom of the press, and in that case, the courts would overrule the government, too. 
Fourth, when the president signs an executive order, these can be overridden by the federal courts.  In fact, there have been a least of few suits of this nature during President Obama's administration. 
Absent a revolution, the mechanisms we have in place are somewhat limited if we seek to overrule some government action or decision.  We can let our representatives know we are unhappy.  We can recall a politician who has made decisions we do not agree with.  We can not vote again for those whom we are displeased with.  We can appeal court rulings or go to court to stop some government decisions and actions. One of the major reasons for a constitutional democracy, though, is to have a rule of law, even to overrule government decisions, so that there is no need for revolution to occur.