The fastest way to discourage individuals from leaving their waste unrecycled is that they be informed it is against the law.
Isn't it better:
...is to inform them it is against the law.
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Sign up to join this communityThe original sentence is not necessarily grammatically incorrect, however it does not sound natural.
It would be better to say something like:
I would also add that "leaving their waste unrecycled" also sounds unnatural. As a native speaker I would personally say something like "the fastest way to discourage individuals from not recycling" or "the fastest way to encourage individuals to recycle"