It is hard to believe that more than a small minority of Korean Christian clergy would provide any support or encouragement for acts of vandalism against the sacred objects of another religion. Moreover, we are unlikely to find written instructions from Protestant clergy to deface Buddhist temples or statues. What we can find is evidence that a few Protestant clergy have incited trouble-makers to perform acts of vandalism. No doubt the vast majority of clergy know their Ten Commandments too well to encourage Christians to perform these acts, but I have compiled some evidence that the Wikipedia claims are probably well-founded.
Barbara O'Brien, an American follower of Buddhism, says ('Christian-Buddhist Tension in South Korea') temple burnings began during the 1980s. In the next few years, through the remaining 1980s and 1990s, several other arsonists destroyed or substantially damaged approximately 20 more Buddhist temple buildings. Vandals often painted red crosses on art and smashed or decapitated Buddha statues.In a few cases Christians, including clergy, were caught in the act but not charged.
Another Wikipedia article about Buddhism in Korea says fundamentalist Protestant antagonism against Buddhism has increased in recent years. Acts of vandalism against Buddhist amenities and "regular praying for the destruction of all Buddhist temples" have all drawn attention to this persistent hostility against Buddhism from Korean Protestants. A Youtube link shows Christians leading a well-organised rally to pray for the destruction of Buddhist temples: not an instruction to vandalism, but some could have taken this as a permission.
The Asia Times (1 Feb 2008) reported that former South Korean President Lee, a Protestant, sent a video prayer message to a Christian rally held in the southern city of Busan in which the worship leader prayed feverishly: 'Lord, let the Buddhist temples in this country crumble down!'" He may not have specifically called for statues to be decapitated, but this would certainly have encouraged acts of vandalism by naive South Korean Christians.
Harold Olsen ('Protestant Priest Urinates on and Defaces Buddhist Shrine') reports that a Protestant minister was caught on a security camera, entering the inner courtyard of a temple in Daegu, where he defaced and urinated on the altar and wall paintings.
So what we do have is evidence that some Protestant leaders are willing to incite their followers, reckless as to the consequences, and some unidentified clergy defacing or burning down temples. At least one member of the Protestant clergy committed minor acts of vandalism.