Molly hasn't used black magic for years, but she still comes awful close to hitting Luccio in Turn Coat, saying something like "I wanted to make her understand." Molly knows it's wrong (morally and legally) to do that, but she still feels the urge, though she thankfully recognized it and stopped it.
I think that is the same thing as the taint.
Removing the taint from this do-gooder goes well, but they still want to help, right? To circumvent that, they either need to voluntarily not use magic in that way, through some kind of extended support group, psychological help, or whatever. Or, someone has to go into their head and mess around until they no longer want to use magic for that purpose. How could they do that without attacking their initial motivation, which was the desire to help those less fortunate? Wouldn't that make them a fundamentally worse person?
I don't think so. With the taint removed, if you explained to them that doing that did more harm than good, if their real motivation was to do good then they wouldn't do it any more.*
But even if not, a clever Blackstaff could get around that... You wouldn't have to attack their motivations at all, just make them not think of using magic in that particular way (exactly like what Mab did to Harry).
The way I see it, the only way to reform a warlock is for them to set aside their magic after removing the taint.
Well, it seems to have worked for Harry... the Winter Mantle temptations have nothing to do with black-magic corruption. He really hasn't shown any signs of that since he got rid of Lasciel's coin, arguably not since well before that (all the anger stuff in PG-WN might have been pure Lasciel influence; I'm not sure anything since GP is confidently attributable to black-magic effects).
It suggests that if you get them early and they really want you certainly can have an effect. All of those falling of the wagon incidents have been discussed here and none of them have been conclusive. They were all open for discussion. And given the stress they are both under I think that is quite remarkable.
Also, every time Molly uses mind magic after PG it seems pretty justifiable, even if technically against the Council's laws (confirming manipulation on Harry in SmF and Luccio in TC*), and Harry was willing in Changes so I don't think that even counts as against the Laws.
*The Council not allowing you to check for that is
flat out suicidal on an organizational level in a world with White Court and a Black Council. Hopefully they've softened on that after TC... If checks for influence were standard for the Senior Council and the Wardens, Peabody could never have done all that damage.