Curses are special Spells only castable by Warlocks through their Patron's Gifts.
Every Curse requires at least 1 Gift, and the corresponding Spirit to cast the Gift.
Only Warlocks are allowed to Curse Targets.
The curser can always decide to drop the Curse as a Reaction. This is not restricted to Line of Sight, or up to discretion of the GM.
A Curse can always be removed, unless otherwise specified, by a Remove Curse of equal or higher level that the Spirit used to cast it. Sometimes Curses require special steps to be removed, and this is up to the discression of the GM.