Possibly.
Presuming this world has magical police (those are going to be necessary in any world with magic), when the witch became trapped in the mirror they, as a perpetrator, were disabled. At this point, force likely to cause grievous bodily harm is not authorized. Similarly, if they had extracted her from the mirror with binding spells capable of holding her, then draining her of her magic was not necessary either - as they had her effectively restrained. Again, force likely to cause grievous bodily harm is not authorized.
If there were a heat of the moment drain - as in, she's hurling fireballs and lightning bolts and in an effort to stop her they drained her, that would be different - as it would be self defense. What you described was not self defense.