"Deactivated" is probably the most technically correct single word answer, but in North America at least, the noun "DEWAT" or "Dewat" is sometimes used also as a verb.
Eg: "That rifle has been deactivated; it is a DEWAT."
is correct usage, but it would also be common to hear something like:
"She had the rifle dewatted because it would be illegal to have a functional one in her collection."
Here is a definition from the BATF in the US:
Deactivated War Trophy (DEWAT) firearms are still firearms under the NFA, >but have been rendered unserviceable (i.e., incapable of discharging a shot >by means of an explosive and incapable of being readily restored to a >firing condition). The deactivation may have been accomplished by various >means such as (but not limited to) welding of the chamber, cutting the >barrel/chamber/breech, plugging the barrel, welding the bolt to the >chamber, or some combination of these actions which rendered the firearm >incapable of firing a shot.
It can also be applied to things like artillery pieces and shells; for instance: http://laststandonzombieisland.com/2014/10/05/want-a-dewatted-bofors-just-25k/.