This situation occurred in the Trisol system. When it's in its extended form, the sophon is extremely vulnerable to nuclear attacks.
Several brilliant fireballs appeared in different parts of the mirror, their blue color distinct from the light reflected from the mirror. These were the exploding nuclear warheads launched by the Trisolaran space defense corps. Because the explosions were happening outside the atmosphere, there was no sound. By the time the fireballs disappeared, several large holes appeared in the mirror, and then the entire surface of the mirror began to tear and crack, until it had broken into more than a dozen pieces.
One assumes that the defence forces on Earth would try much the same thing (e.g. whack it with nukes to see what happens) if a sophon unfolded in space, noting that it's roughly the size of a planet and hence needs to be pretty close by in order to make any significant difference to the amount of radiation being received by the Earth.