The Torah forbids both eating and cooking meat with milk. Thus, one may not cook meat with milk even if he has no intention of eating it. This is clearly indicated by the Torah's formulation in introducing the prohibition of Basar Be'halab (meat with milk) – "Lo Tebashel Gedi Ba'haleb Imo" ("Do not cook a kid in its mother's milk" – Shemot 23:19, 34:26; Debarim 14:21).
Maran (author of the Shulhan Aruch), in his Kesef Mishne commentary to the Rambam's Mishne Torah (Hilchot Ma'achalot Asurot 9:2), posits an intriguing theory in explaining the nature of this prohibition. He contends that the Torah forbade cooking meat with milk as a safeguard against the prohibition of eating meat with milk. As opposed to non-Kosher foods, meat and milk are independently permissible for consumption, giving rise to the concern that if one would cook meat with milk, he might mistakenly partake of the food. The Torah therefore forbade cooking meat with milk, even in cases where one has no intention of eating the final product, as a safeguard against eating meat and milk.
Conceivably, according to this theory, the prohibition of Bishul (cooking) would not apply in situations where there is no possibility of eating the cooked meat and milk. For example, some people have a mechanism in their kitchen sink for garbage disposal. Garbage is placed into the mechanism, and from time to time one activates the system which grinds the garbage and sends it into the home's sewage pipes. It is likely that with time remnants of both meat and dairy products are collected in the system. If one would pour hot water into the sink, he would, in effect, be cooking the meat and milk together. For that matter, in the piping of any kitchen sink there is occasionally a buildup of residue from the utensils washed in the sink, which one might want to eliminate by pouring boiling water into the sink. However, when one pours hot water into the sink, he might very well be cooking the meat and dairy residue together in the pipes.
While instinctively we might forbid pouring hot water down the kitchen sink, the aforementioned theory of the Kesef Mishne might yield a different conclusion. Since the residue in the pipes obviously has no possibility of being eaten, the prohibition of cooking meat and milk perhaps does not apply.
Hacham Ovadia Yosef (in Yalkut Yosef – Yore De'a, p. 174) indeed rules leniently in this regard, invoking the Kesef Mishne's theory as one among several factors leading to his conclusion. In addition to the Kesef Mishne's position, Hacham Ovadia also notes that according to some views, "Iruy Keli Rishon" (hot water poured from the original utensil) does not have the capacity to "cook" in the Halachic sense of the term. These Rishonim (Medieval Halachic scholars) held that hot water can effectuate cooking only while it is still in its original utensil, and not after it is poured. Hence, the hot water poured into the drain does not "cook" the meat and milk residue as far as the prohibition of Bishul is concerned. Although we generally do not follow this view, it may be invoked along with other considerations to allow pouring hot water into the sink. Hacham Ovadia points to other factors, as well, including the fact that it is doubtful whether the hot water will indeed encounter meat and milk at the same moment and cook them together.
Nevertheless, Hacham Ovadia advises one to avoid this issue and not pour boiling water down the drain. If the drain is clogged, one should preferably use a chemical to eliminate the residue in the piping, rather than boiling water.
A similar issue arises in garbage cans. Hacham Ovadia rules that one should not pour hot gravy into a garbage bin that has remnants of dairy foods, such as pizza. Even though this meat and milk mixture will obviously not be eaten, one should nevertheless be stringent in this regard and not pour hot meat on top of dairy foods in the trash bin.
Summary: Strictly speaking, it is permissible to pour boiling water into the drain in the kitchen sink, despite the fact that it will "cook" the buildup of meat and milk residue in the pipe. Nevertheless, it is preferable to use chemicals to clean one's pipes, rather than boiling water. It is forbidden to pour hot meat sauce and the like into the garbage if there is dairy food in the garbage.
