Allama ibn Hazm (رحمه الله) in his short treatise an-Nubaz al-kafiya fi akhamil usul ad-deen said:
Contrary to this, the one who goes against agreement of the Companions (رضي الله عنهم أجمعين), and deliberately transgresses it, if there is a proof (for him established) and he was informed and (he) still persists then he would be considered a disbeliever.
Source: an-Nubaz al-kafiya fi akhamil usul ad-deen p 21
And he also said:
"Each person who comes after and he acts in accordance to agreement of the Companions (رضي الله عنهم أجمعين) and doesnt contradict it, is considered a believer. The one who goes against agreement because of lack of knowledge regarding such his words are not taken into account. The one who acts contrary to it, while he knows, and persist in this is a disbeliever."
Source: an-Nubaz al-kafiya fi akhamil usul ad-deen p25