Won't be legal. BATFE says a semi-auto can't fire from an open bolt, too easy to convert to full auto. Anyway, as pointed out, there would be no point in converting a legal machinegun to semi, since it would still legally be a machinegun. In other words, it would still be "on paper" and transfer would still require the tax. But an irreversible conversion would cut the value at least 80 percent, not a very wise thing to do.
Of course, if one should own a STEN that is not registered, it is illegal and converting it to semi cannot and will not make it legal.
Whether the STEN selector is set to semi or full, the gun still fires from an open bolt. In the semi position, the bolt moving forward trips the disconnector and releases the sear to catch the bolt when it comes back.
The STEN firing pin is machined as part of the bolt, and fires the round as the bolt closes ("open bolt firing"). BATFE says a legal semi has to have the bolt close on the loaded chamber, then have a separate firing pin that is driven by a separate part ("closed bolt firing"). That is why it is difficult to make the STEN system into a semi and the parts are expensive.
Jim