Kufr is refusing to accept the truth when one identifies it. So sealing their hearts, locking inside them their falsehood, and placing a seal on their hearing, so that they do not listen to nor accept the truth, is after they have already refused to accept the truth, and indeed after they have insisted on their position for several times. This is already clear from the previous verse of Quran (2:6) where Allah says:
إِنَّ الَّذِينَ كَفَرُوا سَوَاءٌ عَلَيْهِمْ أَأَنْذَرْتَهُمْ أَمْ لَمْ
تُنْذِرْهُمْ لَا يُؤْمِنُونَ
As to those who reject Faith, it is the same to them whether thou warn
them or do not warn them; they will not believe.
So it is not unjust, but they have entered a stage of Dunya where Allah give time to oppressors to increase their oppression so to make more trouble and their dirty inside be manifested more easily in the outside world, and that's when Allah gets their lives and they enter the hell, the worst places of it (people like those who killed Imam Hussain PBUH in Karbala, or Namrud of Ibrahim's era, or Pharaoh who oppressed Israelis, or the international Zionism now, and many others apparently belong to this category).
فَذَرْنِي وَ مَنْ يُكَذِّبُ بِهَذَا الْحَدِيثِ سَنَسْتَدْرِجُهُمْ مِنْ
حَيْثُ لَا يَعْلَمُونَ . وَ أُمْلِي لَهُمْ إِنَّ كَيْدِي مَتِينٌ
So leave Me with whoever denies the Qur'an. We will progressively lead
them [to punishment] from where they do not know. And I will give them
time. Indeed, My plan is firm.
But regarding your question what sort of Kufr is considered in 2:7 (or indeed in 2:6). According to an almost long Hadeeth from Imam Sadiq PBUH, Kufr is five-fold, two among them being Johud (الجحود), the verse here mentioning one of them (consult Tafseer Al-Burhan's Hadeeth under this verse, or this page for other Ahadeeth). the Johud here is believing in nature instead of God (الدّهریون) which only believe in natural processes and e.g. say وَ قَالُوا مَا هِيَ إِلَّا حَيَاتُنَا الدُّنْيَا نَمُوتُ وَ نَحْيَا وَ مَا يُهْلِكُنَا إِلَّا الدَّهْرُ وَ مَا لَهُمْ بِذَلِكَ مِنْ عِلْمٍ إِنْ هُمْ إِلَّا يَظُنُّونَ which is translated into And they say: What is there but our life in this world? We shall die and we live, and nothing but time (or physical processes, or simply nature) can destroy us. But of that they have no knowledge, they merely conjecture which probably many scientists today belong to this category (their knowledge are merely epistemic, at best theories, all based on assumptions and rules which have just been accepted for the lack of a better one to accept instead).
PS. Science in Islam is not bad, but even encouraged, but for it to be used for a benefit, and not as a religion which is the place for Ontic knowledge.
