A word or phrase to define a person declared guilty by society but found not guilty in a court of law

Solution 1:

The word pariah expresses the rejection of an individual by society on the basis of prejudice rather than justice. I am not commenting on the O J Simpson case in any way, just mentioning it since the OP does so :-

An article in the New Yorker (published in 2001, six years after his 1995 acquittal) says of O J Simpson :

Now he wants vindication. Only that can erase the stigma that has transformed him from an American hero into a pariah, living out his days in a pathetic mimicry of his former life.

Merriam Webster :

Pariah : one that is despised or rejected

Th term originally referred to :

A member of a scheduled tribe of South India concentrated in southern Kerala and Tamil Nadu, originally functioning notably as sorcerers and ceremonial drummers and also as labourers and servants, but later increasingly as ‘untouchables’ in insanitary occupations.

OED-3

The questions that all this raises (in any particular situation) are :

• Is society right or wrong in its prejudice ?

• Is the judicial system effective in its acquitting the innocent or condemning the guilty ?

Solution 2:

social outcast

Merriam-Webster

one that is cast out or refused acceptance (as by society)