Because of the "once a firearm always a firearm" logic of the ATF, I am pretty sure you are going to have to cut the receiver to "destroy" it.
Welded plugs don't satisfy them.
There's an ATF document out on how to do this but I can't seem to find it. Maybe someone else has it handy.
Here is the relevent passage from ATF:
Section 2.5 Removal of firearms from the scope of the NFA by modification/elimination of components.
Firearms, except machineguns and silencers, that are subject to the NFA fall within the various definitions due to specific features. If the particular feature that causes a firearm to be regulated by the NFA is eliminated or modified, the resulting weapon is no longer an NFA weapon.
For example, a shotgun with a barrel length of 15 inches is an NFA weapon. If the 15- inch barrel is removed and disposed of, the remaining firearm is not subject to the NFA because it has no barrel.
Likewise, if the 15 inch barrel is modified by permanently attaching an extension such that the barrel length is at least 18 inches and the overall length of the weapon is at least 26 inches, the modified firearm is not subject to the NFA. NOTE: an acceptable method for permanently installing a barrel extension is by gas or electric steel seam welding or the use of high temperature silver solder having a flow point of 1100 degrees Fahrenheit.
Unfortunately if the barrel is less than 18 inches or overall length less then 26 inches it is an NFA item no matter what you do to disable it.