An anti-possession symbol acts as a ward that will protect the wearer from possession by a demon. The anti-possession symbol for demons appears as a pentagram, created as an endless knot inside a circle reminiscent of a sun symbol. The symbol can be warn as a charm, or tattooed onto the body, though tattooing the symbol appears to be a rarity amongst hunters. The only people known to have had the symbol tattooed onto their bodies are Sam Winchester, Dean Winchester, Mary Winchester, Dean Winchester II, Sera Siege, Kevin Tran and Linda Tran.
Like all other wardings, if the symbol is altered in any way, it's protection is negated.
When the Akrida began an invasion of Earth in 1972, Samuel Campbell suggests to his daughter Mary and John Winchester to get a symbol tattooed on them to protect against being possessed by an Akrida. Jack Wilcox was similarly warded in 1957, forcing the Akrida to come after him by attacking his wife instead.
2.14 Born Under a Bad Sign
Following Sam Winchester's possession by the demon Meg, Bobby Singer gives Sam and Dean each a charm amulet with the symbol.
3.12 Jus in Bello
As a horde of demons lay siege to the police station in Monument, Colorado, Dean gives everyone anti-possession charms to wear, and reveal that he and Sam have had the symbol tattooed onto their chests.
4.18 The Monster at the End of This Book
To prove that they are Supernatural superfans, in order to get Carver Edlund's real name and address, Sam and Dean show off their anti-possession tattoos to the publisher Sera Siege. Who in turn shows off her own (on her ass), before giving them Chuck's name and address.
5.02 Good God, Y'All
Ellen Harvelle revealed that her daughter Jo wears an anti-possession charm.
8.02 What's Up, Tiger Mommy?
Kevin Tran and his mother Linda get the symbol tattooed on their arms to protect against Crowley and his demons from possessing them. However, Linda's tattooed was burned off, allowing her to become temporarily possessed by Crowley.
9.02 Devil May Care
Abaddon threatens to "peel off" Dean's "no demon's allowed" tattoo and possess him.
9.06 Heaven Can't Wait
Kevin Tran's tattoo is briefly visible as he draws blood so Crowley can make a call to Hell and make contact with Abaddon.
9.10 Road Trip
In order to aid in removing the angel Gadreel from Sam's body, Castiel burns off Sam's anti-possession tattoo, allowing Crowley to possess Sam and warn and urge him to eject Gadreel from his body.
13.02 The Rising Son
Sam and Dean have the newly born Jack Kline tattooed with warding and an anti-possession tattoo to protect him from demons. However, Jack's Nephilim powers react on instinct and instantly heal his body of the tattoos as soon as they are completed.
14.11 Damaged Goods
Mary reveals to Nick that she has an anti-possession tattoo on her stomach as proof that she can't be used as a meatsuit for the demon Abraxas.
15.20 Carry On
Sam's adult son, Dean, is shown to have the symbol tattooed on his forearm as he is comforting Sam on his death bed.
1.01 Pilot (The Winchesters)
Both John Winchester and Mary Campbell are in possession of the charm -- Mary wears one on a bracelet, while John's charm is connected to a key given to him that opens a Men of Letters Clubhouse. The charm on the key saves John from potential possession by a demon that attacks him to get into the clubhouse.
1.06 Art of Dying
An anti-possession symbol briefly appears as a drawing in Darla's hunter journal.
1.08 Hang On to Your Life
One of the symbols that Brock Billings was researching for protection from Loki was the anti-possession symbol.
Later, Samuel gives John and Mary information on a tattoo that will protect them from Akrida possession.
1.10 Suspicious Minds
It's mentioned that the Monster Club have all warded themselves against Akrida possession, preventing the Akrida from being able to come after them in that manner, something that the Akrida themselves are aware of. In addition, Jack Wilcox reveals that he is similarly warded against Akrida possession which had caused the Akrida to target him for revenge through his wife Dorothea in 1957 instead.
1.11 You've Got a Friend
John instructs Betty Donelon to get the tattoo in order to protect herself from the Akrida.
1.13 Hey, That’s No Way to Say Goodbye
Joan Hopkins burns off Lata's anti-possession tattoo so that an Akrida can possess her.