If you are unable to clear the VPN SA`s using the "vpn tu" command you may want to try using the following commands

vpn shell /show/tunnels/ike/peer/[remote gw ip]
vpn shell /show/tunnels/ipsec/peer/[remote gw ip]
vpn shell /tunnels/delete/IKE/peer/[remote gw ip]
vpn shell /tunnels/delete/IPsec/peer/[remote gw ip] 

The reason to this can be down to a number of issues and bugs with the Check Point software which they supply Hotfix`s for. Further details can be found on the Check Point site.