Professed Muslim, meaning even if one says he is a Muslim, but he is not, you can still not call him a kaafir as an instruction here:
Quran 4:94 - ...Do not accuse anyone who claims himself to be a Muslim of disbelief just for worldly gains.
But let's say they ARE in fact, polytheists. Can we call them names?
Do not say crude words to those who call upon other than Allah, lest they use crude words about Allah in revenge without knowledge. As such we have made the actions of each nation seem pleasing. To their Lord they shall return, and He will inform them of that they were doing. [6:108]
Volume 8, Book 73, Number 125d:
Narrated Abu Huraira:
Allah's Apostle said, "If a man says to his brother, O Kafir (disbeliever)!' Then surely one of them is such. "
Calling another Muslim a kaafir has been listed by scholars as The Fastest Way to Leave Islam
"Whoever attributes kufr [unbelief] to a believer, he is like his murderer.'' (Tirmizi, ch. Iman (Faith); see Arabic-Urdu edition cited earlier, vol. ii, p. 213. See also Bukhari, Book of Ethics; Book 78, ch. 44)
"Nothing expels a man from faith except the denial of that by which he entered into it [i.e. the Kalima].'' (Majma` az-Zawa'id, vol. i, p. 43)
In the hadith Ibn Umar related that the Holy Prophet said: If a Muslim calls another kafir, then if he is a kafir let it be so; otherwise, he [the caller] is himself a kafir.'' (Abu Dawud, Book of Sunna, edition published by Quran Mahal, Karachi, vol. iii, p. 484)
Abu Zarr reported that the Holy Prophet said: No man accuses another man of being a sinner, or of being a kafir, but it reflects back on him if the other is not as he called him.'' (Bukhari, Book of Ethics; Book 78, ch. 44)
"Withhold [your tongues] from those who say `There is no god but Allah' --- do not call them kafir. Whoever calls a reciter of `There is no god but Allah' as a kafir, is nearer to being a kafir himself.'' (Tabarani, reported from Ibn Umar)
Call not the people of your Qibla [i.e. those who face the Ka`ba in Makka for prayer] as kafir.'' (Al-Nihaya of Ibn Athir, vol. iv, p. 187)
"And among the doctrines of the Ahl as-Sunna is that none of the people of the Qibla can be called kafir.'' (Sharh `Aqa'id Nasfi, p. 121)
"He did not call as kafir anyone from among the people of the Qibla.'' (Sharh Mawaqif, fifth part)
He said: "Nothing expels a man from faith except the denial of that which made him enter it.'' (Rad al-Mukhtar, vol. iii, p. 310)
"It is extremely serious to expel a Muslim from the faith.''(Sharh Shifa, vol. ii, p. 500)
"A ruling of takfir against a Muslim should not be given if it is possible to interpret his words in a favourable manner.'' (Rad al-Mukhtar, Book of Jihad, ch. on Apostasy)
"As for statements of takfir found in books of rulings (fatwa), these are not proof if the authors are unknown and the arguments are missing, because in matters of faith, beliefs depend on conclusive proof, and the takfir of a Muslim is attended with troubles of all sorts.'' (Sharh Fiqh Akbar, by Mulla Ali Qari)
Allama Sayyid Jalal-ud-Din wrote: "The takfir of people of the Qibla is itself an act of unbelief.'' (Dala'il al-Masa'il)
Ibn Abu Hamra, a saint, wrote: "It has already been stated that the rule of the Ahl Sunna is that they do not call kafir, or consider as going to hell eternally, anyone who is of the people of the Qibla.''
"The Imams have made it clear that if there is any ground for not issuing takfir, a ruling of takfir should not be made, even if that ground is weak.'' (Raf al-ishtiba `an `ibarat al-ishtiba, p. 4, published in Egypt)
"Some prejudiced persons from the Asharis call the Hanbalis as kafir, and some Hanbalis call the Asharis as kafir. But their calling each other kafir is not right because the belief of the trustworthy Imams of the Hanafis, Shafi`is, Hanbalis, and the Asharis, is that none of the people of the Qibla can be called a kafir.'' (Miftah Dar as-Sa`ada wa Misbak as-Sayyida, vol i, p. 46)
"The generality of the theologians and the jurists are agreed that none of the people of the Qibla can be called a kafir.'' (Al-Mawaqif, printed in Cairo, p. 600)
The famous eighteenth century saint of Delhi, Khawaja Mir Dard (d. 1785 C.E.), wrote: "We do not call kafir anyone of the people of the Qibla, even though he may be following falsehood or novel beliefs in most matters, because the acceptance of the oneness of God, and the affirmation of the prophethood of Muhammad, and the turning to the Qibla, do not expel them from faith as such. So he would be of those who follow later inventions and falsehood from among the Muslims. The Holy Prophet said: `Withhold in the matter of the people of the Qibla, that you do not call them kafir'.'' (Ilm al-Kitab, p. 75)...