There will be a flag for every perfidious person on the Day of Judgment, and it would be said: Here is the perfidy of so and so. (Using translation from Muslim 1736a)  

عَنِ النَّبِيِّ ﷺ قَالَ لِكُلِّ غَادِرٍ لِوَاءٌ يَوْمَ الْقِيَامَةِ يُقَالُ هَذِهِ غَدْرَةُ فُلَانٍ