Kundalini is to be blocked at Brahmarandhra Nadi
The ultimate union and samadhi happens at sahasrara chakra. This location is at the crown of the head. The very upper portion of head is called brahmarandhra. The energy and consciousness must remain there, if they gone beyond, there is no return back to the physical body.
Jnatva sushumnasadbhedam krtva vayum cha madhyagham /
Sthitva sadaiva susthane brahmarandhre nirodhayet//
Staying in the most conducive place, having found out how to penetrate sushumna and move the prana through the middle passage, it should be blocked in the brahmarandhra, the center of higher consciousness state.(Chapter -4, Verse 16).