In kiosk mode it is still possible to add or remove workspaces with the menu accessible by the middle mouse button. It would be great if it was possible to disable this function.
Sorry to bump this one, but this function is quite annoying even for the usual desktop because it's easy to accidentally delete the last workspace (e.g. by a second middle click).
There's an option in master now to disable the add and remove workspace buttons. The commit id is: f12aa431c11f4aac2e9805f68e2f1d5b5424e079 and there's also an option in xfdesktop-settings. Marking as fixed.