Currently it's not possibl to reset an ipv6 neighbor like its done for ipv4 neighbors.
~$ reset ipv6 bgp
Invalid command: reset ipv6 [bgp]
~$ reset ip bgp
Possible completions:
<x.x.x.x> BGP IPv4/IPv6 neighbor to clear
1-4294967295 Reset peers with the AS number
only shows ipv4 neighbors
as a workaround in my case, i use vtysh directly and do it there:
clear bgp 2001:db8::1