You Disable the permission to enter a URL entry so that your users cannot browse any web page they want.
In order to do so, go under the group-policy you are using. In case you are using the default group-policy:
asa5510-2(config)#group-policy DfltGrpPolicy attributes
asa5510-2(config-group-policy)# webvpn
asa5510-2(config-group-webvpn)# functions none
