Duppies are said to dwell in silk cotton trees (Ceiba pentandra). These trees should not be planted too close to a house because the duppies will "throw heat" on people. Cutting down a silk cotton tree will likely also cause the duppies to attack people.
It is said that nailing a horseshoe over the house will repell duppies.
A conjure bag tied up with a black silk ribbon, containing asafoetida (Ferula assafoetida), garlic (Allium sativum), and salt (sodium chloride) is also said to keep duppies away. Duppies despise salt.
Since some of the USA hoodoo Boo Hag Carolina folklore may have been influenced by Jamaican folklore about duppies, I decided to include this protection spell.
Caribbean Archaeology, Silk Cotton Tree, Home to the spirits of the forest, accessed 4/7/12