The term 'wall up' can be solved by two crossword solutions: MURE (4 letters) and IMMURE (6 letters). MURE is a verb meaning to build a wall or other structure with stones, bricks, or other materials. IMMURE is a verb meaning to enclose within or as if within walls, especially for confinement. Both terms refer to the act of constructing a wall, either with physical materials or figuratively, to confine someone or something.