Dolores Umbridge sent two dementorsDementors to kiss Harry Potter illegally. She used a blood quill-aBlood Quill — a dark artifact-atleast — on at least two students -Harry— Harry and Lee Jordan. She was planning to use the Cruciatus-one — one of the darkest curses-onUnforgivable Curses — on Harry to give up Dumbledore's location. She used what she thought was Veritaserum on Harry illegally.
So, why was she not arrested immediately? In fact, it seems that she wasn't even fired from the Ministry when Scrimgeour became Minister. Considering that Dumbledore had Sirius Black exonerated after his death , arresting and convicting Umbridge should have been quite simple. Yet, she was arrested only after his seventh year.