In case you're still looking for a solution to this, from ASDM configure a Portal Access Rule denying all connections.
From within ASDM navigate to Remote Access VPN / Clientless SSL VPN Access / Portal / Portal Access Rules, then create a rule with Rule Priority=1, User Agent=*, and Action=Deny. ...Or from the CLI, enter into global webvpn configuration and use:
portal-access-rule 1 deny code 403 any
As a fallback / defense-in-depth measure, you can also configure the keepout option. From ASDM it's under Connection Profiles (for either of the Client Access menus... doesn't matter which since it's a shared configuration item) and look for the Shutdown portal login page checkbox and the Shutdown notice: text field. ...Or (again at the global webvpn level), configure as: