That’s a really great question, @anon87969570. Usually, I’d say you could burn or destroy the sigil (as part of a breaking ritual) in order to deactivate it- but for when the sigil is activated by burning or destroying? Hmmm 
After a bit of thought, I’ve got two potential methods for you:
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If you know how the sigil was made, you could follow a classic way of undoing something- which is undoing it by retracing the steps it took to create it. You would start with a recreation of the sigil and could “erase” the lines in the same way it was drawn. Any materials used in its creation would need to be gathered again and then offered as replacements, giving them to nature or a trusted deity. This method erases the sigil from existence through undoing the steps it took to create it
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You could also recreate the sigil (and I would suggest using ashes to draw it for the recreation- most ideally, the ashes from when the original sigil was burned, but any ashes will suffice), stating very clearly that this recreation is a vessel for something that no longer has physical form (the original sigil) and then use the recreation in a deactivation ritual
Just my personal thoughts here- I’m looking forward to what the coven has to think about this one!
Thanks as always for a very thought-inducing question, Ron, and I pray that if you are trying to deactivate a sigil that it goes successfully for you! 
Blessed be! 