The verse "The punishment of those who wage war against Allah and His Apostle, and strive with might and main for mischief through the land is execution, or crucifixion, or the cutting off of hands and feet from opposite side or exile from the land...most merciful" was revealed about polytheists. If any of them repents before they are arrested, it does not prevent from inflicting on him the prescribed punishment which he deserves.
{ إِنَّمَا جَزَاءُ الَّذِينَ يُحَارِبُونَ اللَّهَ وَرَسُولَهُ وَيَسْعَوْنَ فِي الأَرْضِ فَسَادًا أَنْ يُقَتَّلُوا أَوْ يُصَلَّبُوا أَوْ تُقَطَّعَ أَيْدِيهِمْ وَأَرْجُلُهُمْ مِنْ خِلاَفٍ أَوْ يُنْفَوْا مِنَ الأَرْضِ } إِلَى قَوْلِهِ { غَفُورٌ رَحِيمٌ } نَزَلَتْ هَذِهِ الآيَةُ فِي الْمُشْرِكِينَ فَمَنْ تَابَ مِنْهُمْ قَبْلَ أَنْ يُقْدَرَ عَلَيْهِ لَمْ يَمْنَعْهُ ذَلِكَ أَنْ يُقَامَ فِيهِ الْحَدُّ الَّذِي أَصَابَهُ