(They will say);"Our Lord! Remove the torment from us, really we shall become believers!"
means, when the disbelievers witness the punishment of Allah, they will ask for it to be taken away from them.
This is like the Ayat;
وَلَوْ تَرَى إِذْ وُقِفُواْ عَلَى النَّارِ فَقَالُواْ يلَيْتَنَا نُرَدُّ وَلَا نُكَذِّبَ بِـَايَـتِ رَبِّنَا وَنَكُونَ مِنَ الْمُوْمِنِينَ
If you could but see when they will be held over the (Hell) Fire! They will say;"Would that we were but sent back (to the world)! Then we would not deny the Ayat of our Lord, and we would be of the believers!" (6;27)
وَأَنذِرِ النَّاسَ يَوْمَ يَأْتِيهِمُ الْعَذَابُ فَيَقُولُ الَّذِينَ ظَلَمُواْ رَبَّنَأ أَخِّرْنَأ إِلَى أَجَلٍ قَرِيبٍ نُّجِبْ دَعْوَتَكَ وَنَتَّبِعِ الرُّسُلَ أَوَلَمْ تَكُونُواْ أَقْسَمْتُمْ مِّن قَبْلُ مَا لَكُمْ مِّن زَوَالٍ
And warn mankind of the Day when the torment will come unto them; then the wrongdoers will say;"Our Lord! Respite us for a little while, we will answer Your Call and follow the Messengers!"
(It will be said);"Had you not sworn aforetime that you would not leave (the world for the Hereafter). (14;44)
Allah says here;
أَنَّى لَهُمُ الذِّكْرَى وَقَدْ جَاءهُمْ رَسُولٌ مُّبِينٌ