You can deactivate the barrel by welding the chamber. If you don't have a welder, take JB weld and fine steel wool and mix them together to make a very hard structural matrix. Drilling the chamber is also an option.
As far as part condition; rough and crude is the nature of STEN parts. Hastily made with sloppy welds, was sort of the brilliance of the firearm. It was cheap, crude and quick to make. Many also saw extensive use, so many parts are worn. British parts often have a crown, BMP or various subcontractor markings. Pakistani parts often have POF. Peter Laidler covers these matters in his book: "The Sten Machine Carbine".
As an aside, many deactivated GI barrels were drilled through the chamber. It's not uncommon to see one that has been reactivated, often by means of a filler weld or drilling and tapping the holes, screwing in a set screw, then welding the remainder of the hole. While not ideal, they can be fired without deleterious effect.