Though you should never say never, blocking a heater vent to redirect warmth inside your house is normally a safe way to save money by increasing energy efficiency. It would require a massive failure of built-in safety systems for the heat to get hot enough inside a blocked vent box to cause any danger of fire. With that being said, several ways exist to go about blocking heat to unused vents in order to siphon it off and increase the heat production in an area where it is needed more.
Most central heating and cooling systems have ducts attached that feed to at least one vent in each room, and sometimes there are several. Whether to cut down on heating costs or simply direct heat from areas it is not needed to locations where it is more desired, blocking off one or more vents in unused rooms or lightly traveled hallways is a perfectly safe alternative. In many cases, it is as simple as sliding a lever on the vent itself, which slides a set of louvers over the opening.
If no sliding covers are on your vents, or the mechanism doesn't work well, at least one company, ArmourDieCutting.com, exists that sells a simple magnetic vent cover you can put over whichever vent you need to close off. You also can build your own cover out of almost any heat-resistant material. Fitted pieces of cardboard wrapped in aluminum foil and held in place with small weights or magnets is one very low-cost solution. Plywood is another option.
Blocking off one or more vents in the house works quite well in increasing the amount of heat in areas where you might need it more. For example, say your guest bedroom sits empty all winter. No need to keep it warm when the heat flow could be increased to rooms that see heavy use through the cold season, such as the living room or kitchen. As long as your heating system is functioning properly, no reason to worry that blocked vents will create a safety hazard.
Keep in mind that it would never be a good idea to block a majority of vents to a heating system. The heater is designed to operate with a certain amount of air circulation. However, covering up one or two outlets in a system that has ten or more offers little chance of danger, though it would be a good idea to monitor the blocked areas, as well as the heater, during the first few days after redirecting the heat to make sure there are no issues. Ultimately, this is a safe modification to make.