Volcanic planets are actually in the process of cooling themselves down already (see bolded answer further down.) In addition, the heat is not from the volcanoes but rather the greenhouse effect. "Venus is hotter due to the greenhouse effect: Venus has an atmosphere about ninety times thicker than that of Earth, and made almost entirely of carbon dioxide, which is one of the gasses that causes the greenhouse effect on Earth." (Source). As such, most of the following keep that in mind.
Draining the atmosphere from the planet by ejecting it would be one way of dealing with it. Since a hot volcanic planet probably has a very thick atmosphere, this might actually help terraform it towards livable conditions. The opposite problem that Mars has.
Pros: Can help terraform a planet towards our own tolerance.
Cons: Difficult to achieve and astronomically expensive.
Steerable micrometers-thick refractive screens could divert a portion of the sun’s energy away from planet, thus cooling the atmosphere. The screens would orbit between the sun and the planet.
Pros: No pollution; can be turned on or off quickly.
Cons: Even using futuristic launching technology, the 20 million metric tons of mesh would cost $4 trillion to deploy
Reflective particles in low orbit reflect sunlight and cool the planet.
Pros: Closer orbit and low manufacturing costs could make dust cheaper to deploy than space shields.
Cons: Costly to deploy and would require frequent replenishment as solar radiation drives dust down to planet.
PARTICLES IN THE STRATOSPHERE
Sulfate or other reflective particles injected at the equator stay aloft in the stratosphere for one or two years, reflecting sunlight and cooling the planet.
Pros: Principle proven by volcanic eruptions; $130 billion price tag is relatively reasonable.
Cons: Increased acid rain, ozone layer damage.
Reflective balloons would bounce a portion of the sun’s energy away from planet before it had a chance to warm the surface or the lower atmosphere.
Pros: Cheaper to launch than space shields or space dust.
Cons: Would require millions of balloons that would eventually fall to planet as trash.
Ships spray salt-water droplets that make ocean clouds more long-lasting and reflective, cooling the planet.
Pros: Pollution free.
Cons: Would take some 5000 salt-water spraying ships, at \$2 million to \$5 million apiece, to counter a carbon dioxide doubling.
Iron particles spread over unproductive parts of the ocean cause photosynthetic plankton blooms. The plankton absorb carbon dioxide. When they die, they carry some carbon to the ocean bottom.
Pros: Some experiments indicated that thousands of metric tons of carbon were absorbed per metric ton of iron.
Cons: Unclear how much carbon is permanently trapped; plankton blooms can poison other sea life.
Simply painting roofs and roads white could cool populated places by reflecting sunlight.
Pros: Paint is cheap.
Cons: A small effect because much of the sun’s energy is absorbed in the air before it reaches the ground; cooling is local and so could make the local weather worse.
Carbon in the atmosphere or in smokestacks is converted to a form that can be stored underground.
Pros: Already being intensely investigated.
Cons: Could be expensive to deploy the technology and store the carbon; carbon reservoirs could leak.
Trees pull carbon dioxide out of the air and use it to form wood.
Pros: Uncontroversial and already accepted under the Kyoto Protocol.
Cons: Most carbon uptake happens only in the early part of a forest’s growth; new forests could compete with agriculture for land and water.
Original Source for the previous ideas modified slightly by myself (added drain atmosphere).