A method of preventing undesired fluid flow to or from a permeable zone (10, 11) in a subterranean production or injection well, wherein the well, at least in a portion where the isolation is to be carried out, is provided with a pipe body, the method comprising the following steps: lowering a perforation tool into the pipe body onto a longitudinal section of the well where the isolation is to be carried out; by means of the perforation tool, forming holes in the pipe body along the longitudinal section; by means of a flushing tool attached to a lower portion of a flow-through pipe string, and lowered into the pipe body onto the longitudinal section, pumping a flushing fluid down through the pipe string, out through at least one flow-through outlet in the flushing tool, into the pipe body and further out, via holes (212, 213) in the pipe body, into an annulus between an outside of the pipe body and a surrounding well body, thereby cleaning the annulus; pumping a fluidized plugging material down through the pipe string and out through the flushing tool, into the pipe body and further out into the annulus via holes (212, 213) in the pipe body; placing the fluidized plugging material along the longitudinal section of the well, after which the plugging material forms a plug covering substantially a complete cross section of the well, whereby the plug fills an inside of the pipe body and the annulus between the outside of the pipe body and the surrounding well body (7, 71); - wherein at least one of the at least one outlet in the flushing tool is angled non-perpendicularly relative to a longitudinal axis of the flushing tool, whereby a corresponding discharge jet from the flushing tool also will be non-perpendicular to the longitudinal axis of the flushing tool. lowering a perforation tool into the pipe body onto a longitudinal section of the well where the isolation is to be carried out; by means of the perforation tool, forming holes in the pipe body along the longitudinal section; by means of a flushing tool attached to a lower portion of a flow-through pipe string, and lowered into the pipe body onto the longitudinal section, pumping a flushing fluid down through the pipe string, out through at least one flow-through outlet in the flushing tool, into the pipe body and further out, via holes (212, 213) in the pipe body, into an annulus between an outside of the pipe body and a surrounding well body, thereby cleaning the annulus; pumping a fluidized plugging material down through the pipe string and out through the flushing tool, into the pipe body and further out into the annulus via holes (212, 213) in the pipe body; placing the fluidized plugging material along the longitudinal section of the well, after which the plugging material forms a plug covering substantially a complete cross section of the well, whereby the plug fills an inside of the pipe body and the annulus between the outside of the pipe body and the surrounding well body (7, 71); - wherein at least one of the at least one outlet in the flushing tool is angled non-perpendicularly relative to a longitudinal axis of the flushing tool, whereby a corresponding discharge jet from the flushing tool also will be non-perpendicular to the longitudinal axis of the flushing tool. - wherein at least one of the at least one outlet in the flushing tool is angled non-perpendicularly relative to a longitudinal axis of the flushing tool, whereby a corresponding discharge jet from the flushing tool also will be non-perpendicular to the longitudinal axis of the flushing tool.