You have some substance or conditions that prevents you from working magic. If this pheoomena is even near, you become almost helpless. The severity of this weakness depends on how commonly available the phenomena is; more unusual substances offer greater protection.
This can be a set of substances, whose rarity is decided by the whole set. Nor does this have to be a substance in the strict sense of the word - any phenomena, such as certain sigils, a song, a type of animal or certain temperatures, work just as well.
A common substance - like iron or steel in a modern world- only stops your powers if you are wearing significant amounts of it or sitting inside a contraption made of it (most vehicles are ferrous).
An unusual substance, such as lead or salt, will prevent you from using your magic even in small quantities. A stunt can be made to place the substance on your person.
A rare substance, such as meteoritic iron or any radioactive isotope, will prevent you from working your magic if it is on your person, or if it is on the person of your target or in your line of sight to that target.
A unique substance, or one that is very hard to acquire, such as kryptonite or the staff or Amunhotep, will prevent you from using your sorcery as long as it is in the vicinity. This level of scarcity is mostly a plot device for villains.