I would like to block connections that are still using old versions of the VPN client software.
I use an ASA5510.
I can ask clients to use the new version as provided on the ASA but they can still refuse this.
To force the use of the latest client I will have the abillity to block the older versions.
You can restrict the VPN Client versions connecting to the asa using the "client-access-rule" in your group-policy attributes. With this command you can restrict by type or version of the client.
You'll find the details on how to use it in the following link, so you can restrict the old versions you want to avoid:
Hope that works for you!