Banishing the Witnesses was a spell to put the Witnesses back to rest.
As part of the 66 Seals to bring on the Apocalypse, Lilith cast the Rising of the Witnesses. This raised ghosts who had been killed by supernatural means and who hunters had failed to save. They rose as rabid animals due to the manner and circumstance of their deaths and tried to kill the hunters who had failed to save them. After discovering what was going on, Bobby Singer and Sam and Dean Winchester decided to use this spell to put the Witnesses back to rest, and had to emerge from Bobby's Panic Room to do it as they needed his fireplace and ingredients not in the room. As Bobby worked to cast the spell, Sam and Dean had to defend him from the ghosts of Ronald Reznick, Victor Henriksen, Meg Masters and two little girls Bobby had failed to save from an unknown monster. Sam and Dean mostly succeed in keeping the ghosts off of Bobby, but just before he could complete the spell, Meg noticed what he was doing and tried to kill him. Dean grabbed the bowl with the ingredients and at Bobby's command, threw them into the fireplace, completing the spell and banishing the Witnesses back to their rest. However, it was too late for 20 other hunters elsewhere, according to Castiel.
The spell required a sigil to be drawn on a flat surface, and a mixture of opium, wormwood, hemlock, a hex box full of unknown ingredients and an open fire. The caster was required to crush the ingredients together in a bowl while reciting the spell, without interruption, or it would fail. Once the recitation was done, the ingredients had to be dumped onto the fire. Once the ingredients were set aflame, they generated a blue pulse of energy that sent all the Witnesses back to rest.
Dean identified wormwood as the plant associated with the restless dead in "Houses of the Holy". It is also the name of the star signifying the arrival of he The Horseman War in the book of Revelations.