on your tunnel-group webvpn-attributes configuration you should see some group-alias attached to it.
If you don't want a specific alias in this case webmail then you should be able to remove the line from the configuration.
That will prevent users from connecting to that group on the drop-down since it will not show anymore.
For example:
tunnel-group SSL webvpn-attributes
group-alias webmail enable
group-alias connect enable
tunnel-group SSL webvpn-attributes
no group-alias webmail enable