Assuming your guest wifi is 10.2.2.0/24 and the specific address is 10.1.1.100/24
set interfaces vlan unit 999 description "guest wifi" set interfaces vlan unit 999 family inet filter input guest-outbound-filter set interfaces vlan unit 999 family inet filter output guest-inbound-filter set interfaces vlan unit 999 family inet address 10.2.2.1/24 set firewall family inet filter guest-inbound-filter term empvlan from source-address 10.1.1.100/32 set firewall family inet filter guest-inbound-filter term empvlan then accept set firewall family inet filter guest-inbound-filter term deny-everything-else then discard set firewall family inet filter guest-outbound-filter term empvlan from destination-address 10.1.1.100/32 set firewall family inet filter guest-outbound-filter term empvlan then accept set firewall family inet filter guest-outbound-filter term deny-everything-else then discard
Assuming you want guest wifi to also have internet access:
set interfaces vlan unit 999 description "guest wifi" set interfaces vlan unit 999 family inet filter input guest-outbound-filter set interfaces vlan unit 999 family inet filter output guest-inbound-filter set interfaces vlan unit 999 family inet address 10.2.2.1/24 set firewall family inet filter guest-inbound-filter term empvlan from source-address 10.1.1.100/32 set firewall family inet filter guest-inbound-filter term empvlan then accept set firewall family inet filter guest-inbound-filter term no-rfc1918 from source-address 10.0.0.0/8 set firewall family inet filter guest-inbound-filter term no-rfc1918 from source-address 172.16.0.0/12 set firewall family inet filter guest-inbound-filter term no-rfc1918 from source-address 192.168.0.0/16 set firewall family inet filter guest-inbound-filter term no-rfc1918 from source-address 169.254.0.0/16 set firewall family inet filter guest-inbound-filter term no-rfc1918 from source-address 127.0.0.0/8 set firewall family inet filter guest-inbound-filter term no-rfc1918 from source-address 224.0.0.0/3 set firewall family inet filter guest-inbound-filter term no-rfc1918 then discard set firewall family inet filter guest-inbound-filter term allow-internet then accept set firewall family inet filter guest-outbound-filter term empvlan from destination-address 10.1.1.100/32 set firewall family inet filter guest-outbound-filter term empvlan then accept set firewall family inet filter guest-outbound-filter term no-rfc1918 from destination-address 10.0.0.0/8 set firewall family inet filter guest-outbound-filter term no-rfc1918 from destination-address 172.16.0.0/12 set firewall family inet filter guest-outbound-filter term no-rfc1918 from destination-address 192.168.0.0/16 set firewall family inet filter guest-outbound-filter term no-rfc1918 from destination-address 169.254.0.0/16 set firewall family inet filter guest-outbound-filter term no-rfc1918 from destination-address 127.0.0.0/8 set firewall family inet filter guest-outbound-filter term no-rfc1918 from destination-address 224.0.0.0/3 set firewall family inet filter guest-outbound-filter term no-rfc1918 then discard set firewall family inet filter guest-outbound-filter term allow-internet then accept