I think, in the Rashi on Sanhedrin 44b, the subject of the sentence "He should lift one of them from the ground" is not referring to one of the witches, but to one of the jars that he distributed in the sentence before.
As translated here: http://www.bmv.org.il/shiurim/sanhedrin/san080.html
[Shimon ben Shetach] assembled eighty tall young men and distributed to each of them a jar
with a cloak wrapped up inside (it was a rainy day). He also told them
to make sure that they were always eighty in number. "When you come
inside," he said, "one of you must raise his jar from the ground; from
that moment the witches will have no further hold over you; if that
does not work then we can never beat them." Shim'on ben-Shataĥ went
into the witches' coven and left the young men outside. When the
witches asked him who he was he replied that he was a wizard who had
come to test them with his wizardry. "What tricks can you do?" they
asked. "Despite the fact that it is raining today I can produce eighty
young men with dry cloaks!" "Show us!" He went outside and beckoned
the young men inside. They removed the cloaks from the jars, put them
on, and came into the coven. Thus they bettered the witches, took them
outside and strung them all up.
I don't know how lifting one of the jars from the ground would render their powers useless, but maybe that answer lies in what power the witches claimed to have.