It may be reversible if you are prepared to commit a criminal offence, manufacturing a prohibited device.
The neatest conversion I have seen involves the magazine follower spring guide. It is replaced with one that restricts movement of the magazine follower. This piece is a permanent part of the magazine assembly. This allows the magazine to be unloaded from the bottom.
Another method is to drill a hole through the inner magazine, insert a pin, and weld its ends. The magazine cannot be swung open if this is done, nor can the magazine be unloaded from the bottom.
I altered a number by welding a plate to the side of the inner magazine box. this would limit the number of cartridges that the magazine could hold, but the magazine could still be swung open, and unloaded from the bottom.
There have also been blocks welded or rivetted to the inside of the outer magazine casing.