A method of preventing undesired fluid flow to or from a permeable zone 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: (A) lowering a perforation tool into the pipe body onto a longitudinal section (L1) of the well where the isolation is to be carried out; (B) by means of the perforation tool , forming holes in the pipe body along the longitudinal section (L1); (C) 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 (L1), 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 in the pipe body , into an annulus between an outside of the pipe body and a surrounding well body, thereby cleaning the annulus ; (D) 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 in the pipe body ; (E) placing the fluidized plugging material along the longitudinal section (L1) of the well , after which the plugging material forms a plug covering substantially a complete cross section (T1) 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 ; - 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 . (A) lowering a perforation tool into the pipe body onto a longitudinal section (L1) of the well where the isolation is to be carried out; (B) by means of the perforation tool , forming holes in the pipe body along the longitudinal section (L1); (C) 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 (L1), 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 in the pipe body , into an annulus between an outside of the pipe body and a surrounding well body, thereby cleaning the annulus ; (D) 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 in the pipe body ; (E) placing the fluidized plugging material along the longitudinal section (L1) of the well , after which the plugging material forms a plug covering substantially a complete cross section (T1) 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 ; - 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 . <EOS>