Just create a VPN firewall rule;
- Security & SD-WAN > Configure > Site-to-site VPN
- Under "Organization-wide settings"
- Add a "Site-to-site outbound firewall" rule
- Policy: Deny
Protocol: Select UDP or TCP
Source: any
Src port: any
Destination: any
Dst port: 50001
Do the same for "Site-to-site inbound firewall" rule too.