I have seen that when one of our community elder died and when they put on tallit they cut out the tzitzit part and let him put only the garment. Why is this done to a dead person? Is there a source for this and a good reason ?
The source is from Shulchan Aruch Yoreh Deah 351:2
אֵין קוֹבְרִין אֶת הַמֵּת אֶלָּא בְּטַלִּית שֶׁיֵּשׁ בּוֹ צִיצִית. הַגָּה: וְיֵשׁ אוֹמְרִים דְּאֵין צָרִיךְ צִיצִית (טוּר בְּשֵׁם י''א). וְנָהֲגוּ לְקָבְרוֹ בְּצִיצִית, אַךְ שֶׁפּוֹסְלִין תְּחִלָּה הַצִּיצִית, אוֹ כּוֹרְכִין אֶחָד מִן הַכַּנְפוֹת.
The Shulchan Aruch writes that one buries a dead in a talit with tzitzit. The Tur writes that some say one doesn't need tzitzit and the Rema rules that one uses a talit but first invalidates one of the tzitzit.
JVL comments this is indicating the deceased is no longer required to perform rituals.