How to disable Access Gateway Window Cleanup Dialog box pop up when the SSL VPN connection gets terminated?

Cleaning Up Your Computer when Your Session Ends

During a session, temporary files are created on your computer. The Access Gateway has a cleanup feature to remove temporary files from your computer. This feature is enabled by your system or network administrator.
You can select the following items for cleanup, which removes them from your computer: 
Access Gateway Plug-in w Certificates used for authentication.
Applications used during the session.
Passwords and autocomplete data.
History and Web addresses typed in the address bar.
Cookies and temporary files

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Instructions

Please run the following command to disable Client Cleanup Prompt 

Set vpn parameter clientcleanupPrompt OFF

Also, if you would like to disable this using GUI, then the option is available in Global Settings under the Client Experience tab.