What is a correct translation for 「そんなことない」 and when can it be used?
I understand the "this is wrong"/"this is not correct" general sense but is this considered either as harsh or a gentle way of showing disagreement?
Also a direct translation would be "there is no such thing". Can this expression be applied to a person?
It is relatively gentle, and a good way of allaying fears or dispelling misconceptions.
お邪魔みたいですので、これで失礼しますね… It seems like I'm interrupting you, so I'll see myself out... そんなこと（は）ないですよ Not at all! / Don't be silly! / No such thing!
かなり怖い人だそうですけど… I heard he's quite a scary person...
そんなことない Not at all.
If by your last question you mean can the "thing" be a person, I would say no. When you start making the "thing" a literal object instead of a fact, そんなものはない (there is no such thing [here]) fits better, and for people, そんな人はいない (there is no such person [here]).
