Radak, Targum Yonasan: This is the line of the Churban, like "v'Natah Aleha Kav Sohu" (Yeshayah 34:11).

Malbim: Kav is [a stick] used to measure the length and width of a wall. Hashem says, metaphorically, that he extended a Kav against Shomron to destroy it from under, and against Beis Achav from above, i.e. it has no share in the world to come.

Rashi: It is a string (plumb line) to which people building a wall tie lead (so it will go straight down), to make the wall straight.

Rashi: He cleans the bowl. Radak: "Ish" is omitted. (Like a person empties a bowl, i.e. he turns it over.)

Radak: When one cleans a bowl (Malbim - and removes everything stuck to it), afterwards he turns it over, so it will not get dirty again (Malbim - so nothing else will be put in it. So He will remove all the nation in Yerushalayim and exile them, and then He will destroy it, so it will not be possible to live there.)