Sometimes after you go through the process of casting a sigil you may want to know how to undo it in order to nullify the effects that it is giving off. In order to go through this process you can do one of two things you can:
Destroy the sigil, and allow its energy, and intention to run its course, and eventually nullify itself. You would be able to help out this nullification process by taking away the energy that you gave it in the charging process, while also focusing on its intention that is imprinted in your subconscious, changing it into what you want to manifest, allowing the sigil to nullify quicker and more effectively.
The other thing you could do is send out another sigil with the opposite intention, or with the intention of stopping the first sigil, so that it can go through the process of nullifying its effects.
Both of these methods will be good to know, and learn in case you ever want to stop a sigil for any reason. These also work on sigils that have been cast by other practitioners, and may be very helpful in defending from spiritual attacks.