How can I stop the pop up asking for permission for the web update client?

Prism 5 Windows tries to check for updates using a module called Prism 5 Web Update Client. Sometimes this triggers a Windows dialog box:

Whether or not this dialog comes up depends on your setting in the Windows control panel. Click Yes, and the dialog goes away.

The last version of Prism 5 Windows was 5.04, and we will not release any more updates to Prism 5. Therefore it makes sense to tell Prism to stop checking. You'll find this option at the bottom of the Internet tab of the Preferences dialog. Uncheck the option, and Prism will stop looking for updates so you won't see that dialog (from Prism) any more.

Of course, you can always ask Prism to check for the presence of updates by dropping the Help menu and choosing Check for Updates.