Sh. Ibn al-ʿUthaymīn رَحِمَهُ اللهُ said, “Yes, it is permissible for us to declare a specific person to be a disbeliever if the causes of disbelief are confirmed. Hence, if we see a man rejecting messenger-ship (al-risālah), or a man who permits seeking judgement from a false deity (ṭāghūt), or a man who permits ruling by other than what Allāh revealed, and he says it is better than the ruling of Allāh after the proof has been established against him, then we judge him to be a disbeliever. Thus, if we find the causes of disbelief, and the pre-requisites are confirmed, and the preventative factors are removed, then we excommunicate the person specifically, and we make it binding on him to return to Islām, or he is executed.”

Source: Majmūʿ Fatāwá wa Rasāʾil Faḍīlat al-Shaykh Muḥammad b. Ṣāliḥ al-ʿUthaymīn, 2: 124-125, no. 218.