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Yisrael Kedoshim Heim: Destroying even Chametz that is allowed to be owned: [1]
Although there were cases explained above in which from the letter of the law one is not required to clean the Chametz, nevertheless, the Jewish people are holy [i.e. Yisrael Kedoshim Hem] and are accustomed to be stringent upon themselves and scrape off all Chametz, even a mere speck, that is stuck to the walls/ceiling/floor or a vessel. They are even furthermore stringent to sand down the benches and chairs and walls which have touched Chametz. [In light of this stringency] if there is Chametz [in an area that one cannot reach, such as] in a crack that one cannot remove, then one can place cement over it [or anything else something to make it not fit for eating such as bleach].[2]
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[1] Admur 442:30; Michaber 442:6 “The custom is to scrape the walls and chairs which touched Chametz and they have upon whom to rely”; Tur 442; Rosh 3:2 “I did not lengthen on these laws of dough stuck on vessels as the Jewish people are holy and are accustomed…”; Raavan, brought in Rosh ibid “This custom of scraping down the walls and chairs has a root in the Yerushalmi”; Rokeiach 247; Radbaz 1:135 “The Jewish people are holy as writes the Rosh, and as we see that they keep extra Chumros, in contrast to other Issurim”; Yosef Ometz 699; Mamar Mordechai 442:6, M”B 442:28, Kaf Hachaim 442:69 “Since the custom is based on the Yerushalmi, one is therefore not to belittle it and claim it is a Minhag Shtus and superfluous stringency.”
[2] Admur ibid; Michaber ibid; From the letter of the law however, it is permitted to leave the Chametz there throughout Pesach, so long as one plans to do Bittul. [See Admur 433:19] Lit. Tit; To note that this custom has Halachic significance as is seen from chapter 444, that even on Shabbos which is Erev Pesach one may wash/scrape off even less than a kezayis of Chametz from a vessel, even though from the letter of the law it does not need to be removed.