The Istari were forbidden to "match [Sauron's] power with power". Yet the White Council was able to attack Dol Guldur and drive out Sauron. Why was that allowed? Or did the Istari just help out with the planning, and the Elves of the White Council handle the actual attack?
Unfortunately, we're given too few details of the actual attack and to what extent were the Istari involved. Hence we have no idea whether the Istari were directly involved in the battle, or if they were just the behind-the-scene planners.
It's very likely to be the latter. Gandalf, after the War, was said to be the "mover of the events of the War" by Aragorn. Rightly said, as Gandalf was the one who got Frodo (and Bilbo) on his journey. That means that it's very likely that in the discussions of the White Council, Gandalf merely urged the Council to attack Dol Guldur, but didn't go into battle himself.
All we do know for sure is that their victory was achieved by the "devices of Saruman" (whether this refers to weapons, magic or some sort of strategy is not clear).
Peter Jackson's Hobbit movie(s) clearly intrepreted this wrongly by featuring Gandalf fighting the Necromancer as he gets captured and Saruman fighting the Ringwraiths at Dol Guldur.
To answer your question: they couldn't, and even if they did, we have no idea.
In Unfinished Tales we are told
And this the Valar did, desiring to amend the errors of old, especially that they had attempted to guard and seclude the Eldar by their own might and glory fully revealed; whereas now their emissaries were forbidden to reveal themselves in forms of majesty, or to seek to rule the wills of Men or Elves by open display of power, but coming in shapes weak and humble were bidden to advise and persuade Men and Elves to good, and to seek to unite in love and understanding all those whom Sauron, should he come again, would endeavour to dominate and corrupt.
The Istari were banned from ruling Elves and Men through open displays of power, not from using their power against Sauron.
The Lord of the Rings has a simplified version of this.
It was afterwards said that they came out of the Far West and were messengers sent to contest the power of Sauron, and to unite all those who had the will to resist him; but they were forbidden to match his power with power, or to seek to dominate Elves or Men by force and fear.
Here I personally interpret this as meaning they are forbidden from becoming rulers and building a power base of their own. As Treebeard says of Saruman in Lord of the Rings
‘I think that I now understand what he is up to. He is plotting to become a Power. He has a mind of metal and wheels; and he does not care for growing things, except as far as they serve him for the moment.
Sauruman falls because he plots to become a Power, and rule in his own name and not for the freedom of Elves and Men and the wishes of the Valar.