You probably know already that words in German can be put together - German is for languages what Lego is for toys.
"reißen" means "to rip" or "to yank" and any composite with "reißen" reflects that: einreißen (to tear down, for walls), abreißen (to rip off, also to tear down, but for buildings), auseinanderreißen and zerreißen (to rip to pieces, to rip/tear apart), zurückreißen (to yank back), and so on.
"herunter", on the other hand, means "down" or "downwards" with the connotation of "in the direction towards the observer" (away from the observer would be "hinunter").
From this alone one can conclude that "herunterreißen" is a valid word, but not for tearing down walls or buildings. You can use it for not taking but ripping off e.g. pictures from a wall or for removing wallpaper (that also requires some force). "Reißen" always includes an element of force and "herunter" is a movement towards you. (Corollary: "hinunterreißen" exists too and is something a waterfall does when it takes something down with it - it "yanks it downwards", away from you.)
Alternatives for "einreißen" - tearing down walls - are "durchbrechen" (not necessarily removing the whole wall but maybe just create a door or window in it), niederreißen (works for both buildings and walls) and - for larger structures like city walls, but also for buildings - "schleifen". Notice that there are two words "schleifen": schleifen-schliff-geschliffen, which means to sharpen and schleifen-schleifte-geschleift, which, depending on context, means either to drag or to tear down a building or wall.