First clear phase 1 (IKE):

clear security ike security-associations

then clear phase 2 (IPSEC):

clear security ipsec security-associations

First clear phase 2 (IPSEC):

clear security ipsec security-associations index INDEX-NR

then clear phase 1 (IKE):

clear security ike security-associations IP-NUMBER

https://forums.juniper.net/t5/SRX-Services-Gateway/How-is-the-fastest-way-to-disable-or-reset-a-ipsec-peer/td-p/87366