By default on an R77.30 gateway, you cannot disconnect a user from an SSLVPN. You need to enable the Session Visibility and Management Utility.
To enable the feature you need to run the command RAsession_util on, followed by cpstop/cpstart. This needs to be done on all cluster members.
Details on the above and how to view\disconnect users can be found in the R77 Mobile Access support document on page 167 under the Session Visibility and Management Utility header, and under sk104644.