Group locking, as Cisco call it, has been available since ancient IOS 12.2(13)T (circa 2003) and still – most of the set ups I see of clients’ VPN servers at most use different VPN groups for different privilege access requirements and blissfully ignore the fact that all it takes to get more enabled access is to know the pre-shared key of the other VPN group. And believe me – it is not that hard when group pre-share key (PSK) is known to half of the company. So if you happen to stumble on this post bear with me and let’s fast forward from accepted practices of 90’s to 2010.
Below are possible ways to lock users connecting to Cisco device (IOS router and ASA to be precise) to predefined VPN groups and do it forcefully so that even if the end user knows the PSK of other VPN group(s) she won’t be able to connect with it.

Case 1. Cisco IOS router acting as Ezvpn server , users are authenticated locally by the router. Let’s name it – group is JUNIPER , and the local user is John.Chambers and we want to confine this user to this group for ever.
Enable group locking for specific group (don’t forget to do the same for all VPN groups)

R1(config)#crypto isakmp client configuration group JUNIPER

Now restrict user to be able to use this group only. For that you have to reconfigure user to look like username followed by delimeter (that can be any of @, %, /, \) and then group name , to be concrete

R1(config)#username John.Chambers@JUNIPER secret Idontworkforsalaryanymore

from now on user John.Chambers will be able to authenticate with Cisco only using John.Chambers@JUNIPER . It overrides any user for VPN connection that already exists, that is if there is already user John.Chambers it will not be able to connect with the group JUNIPER . On the other hand anyone getting PSK of the VPN group JUNIPER will fail authentication if the user is not explicitly reconfigured in the new format.
Case 2 . Cisco IOS router users are authenticated using external Radius server. Unlike local authentication, with Radius you create the user as usual – John.Chambers but then assign it in the Settings cisco-av-pair attribute called user-vpn-group, like this:
Case 3.ASA Local username authentication.
No fancy username/group configuration here, you just lock username to a group under general attributes of the user.

ASA1(config)# username John.Chambers password Idontworkforsalaryanymore
ASA1(config)# username John.Chambers attributes
ASA1(config-username)# group-lock value JUNIPER

Case 4. ASA Radius authentication .
Here also the VPn group is forced for the user settings using the following attribute:
[3076\085] Tunnel-Group-Lock JUNIPER