Often branches of hawthorn were used for such purposes. One example of this is given in Vampires, Death, and Burial by Paul Barber (Yale University Press, 1988):

"In Eastern Serbia, a small hawthorn peg may be driven into the grave, beside the cross, to prevent the corpse from becoming a vampire."

Thorns (along with other sharp objects like nails and knives) can also be inserted under a corpse's tongue to prevent it from sucking blood. (Barber, 1988, 52) Cremating a vampire with wood taken from thorn bushes also destroys them very effectively. (Barber, 1988, 64)

Thorns, however, were used for other reasons than their ability to prick and draw blood. They also have a mystical power to deter evil.

"The branch of wild rose on his coffin keep him that he move not from it..."
- Professor Van Helsing in Mina Harker's Journal, Chapter XVIII of Dracula by Bram Stoker.

Stoker's source for this was probably the book about Transylvania by Emily Gerard.

Montague Summers wrote on page 309 of his book The Vampire in Europe, first published in 1928, that:

"...on the Eve of St. George's Day in Transylvania there used not to be a Saxon farm in Transylvania which had not the gates of the yard decorated with branches of wild rose bushes in order to keep out the witches."

In the book The Lore of the Forest by Alexander Porteus, first published in London in 1928 by George Allen & Unwin, it is written:

"Thorns, thistles, etc., are credited with having a certain magic power owing to their capacity to lay hold of a thing. On Walpurgis Night [April 30 - the Eve of May Day], the night on which all witches met to hold their unholy revels, it was customary in Bohemia to place branches of hawthorn, gooseberry, wild rose, and other prickly plants on the thresholds of the cow-houses in order to catch the witches and prevent them from entering."

 

Origin

The crown of thorns, a symbol Christ's suffering for man, was particularly relevant in the power of the thorn in keeping away evil.

But the general belief is older than Christianity as Pliny, in 77 A.D. refers to thorns as being auspicious at weddings.

In his book Fasti, the ancient pagan Roman author Ovid gives a tale where a branch of white thorn, a species of hawthorn, is placed in a window to prevent the blood-sucking striges from preying upon an infant.