Polytheism, idol worship or shirk
Worshipping others along with Allah is an unforgivable sin.
Verily, whoever sets up partners in worship with Allah, then Allah has forbidden Paradise for him, and the Hellfire will be his abode (Quran 5:72)
Worshipping idols, statues and pictures are forbidden.
You worship besides Allah only idols, and you only invent falsehood. Verily, those whom you worship besides Allah have no power to give you provision. (Quran 29:17)
Relying on intercession from intermediaries (such as saints) is a form of idolatry and takes one out of Islam.
So no intercession of intercessors will be of any use to them. (Quran 74:48)
Grave worship and seeking help from the dead is forbidden in Islam.
"Those before you took the graves of their Prophets as places of worship. Do not take graves as places of worship for verily, I forbid you to do so." (Hadith)
Magic and sorcery in all its forms is forbidden.
But neither of these (two angels) taught anyone (such things) without saying, "We are only for trial, so disbelieve not (by learning such magic)." (Quran 2:102)
Amulets and charms are forbidden in Islam.
"Spells, charms and love-potions constitute shirk." (Hadith)
Fortune-telling and palm-reading is forbidden in Islam.
He is the Knower of the unseen, and He does not reveal His secrets to anyone but whom He chooses (of His Prophets and Messengers). (Quran 72:26-7)
Astrology and Horoscopes are forbidden in Islam.
"Whoever acquires knowledge of any branch of astrology, has acquired knowledge of a branch of magic. The more he increases in that particular knowledge, the more he increases in sin." (Hadith)
Muslims never approve of shirk and avoid those who practice shirk and their actions.
And when (the hypocrites) meet those who believe, they say: "We believe," but when they are alone with their evil ones, they say: "Truly, we were with you; verily, we were but mocking. (Quran 2:14)
Making fun of and rejecting Allah's Orders is forbidden in Islam.
And he who does not rule by what Allah sent down, it is they who are the disbelievers. (Quran 5:44)