What is the ruling on one who describes previous laws revealed by Allah as "void," while believing in and respecting them? And is this considered defamation or belittling of a religion revealed by Allah?

If the intent behind stating the invalidity of previous laws is that they have been abrogated and are no longer permissible to worship by, then this is correct. However, it is more appropriate to use the term "abrogation" (naskh) rather than "invalidity" (butlan). Muslims are in agreement that all previous laws have been abrogated by the law of our Prophet Muhammad, peace be upon him, based on the Almighty's saying: "And We have sent down to you the Book in truth, confirming that which preceded it of the Scripture and as a guardian over it. So judge between them by what Allah has revealed." No Muslim can claim that those laws were invalid at the time of their revelation. We advise against indulging in whisperings (waswas) and extremism.