@Subir Saha Thank you for reaching out to us. You can fetch the VpnConnectionId by running the command mentioned here
Get-AzVirtualNetworkGatewayVpnClientConnectionHealth -VirtualNetworkGatewayName <name of the gateway> -ResourceGroupName <name of the resource group>
Copy the VpnConnectionId of the session that you want to disconnect.
Disconnect-AzVirtualNetworkGatewayVpnConnection -VirtualNetworkGatewayName <name of the gateway> -ResourceGroupName <name of the resource group> -VpnConnectionId <VpnConnectionId of the session>
You can try this via Portal if PowerShell command fails. Hope this helps! Please let me know if you are still facing the same issue. Thank you!