When you right click the workspaces in the bottom right, and goto properties. You can disable the switching of workspaces with the mouse wheel when the mouse is over the workspaces area. It would be beneficial I'd guess to also make available the option to disable the same function with the mouse over the desktop. Currently its in Window Manager Tweaks > Workspaces.
Because the option is in different programs, it is currently not possible to do this. I not sure it will be in the future, but maybe when we get a new configuration system for Xfce 4.6. I'll leave the report open, but don't expect it to change anytime soon, sorry. Thanks for reporting it.
Is there any news on this one. In XFCE 4.10. there is an option to enable/disable the switch workspace with mouse wheel in the Workspace Switcher panel plugin, but it don't seams to do any thing. Only the setting in "Windows Manager Tweaks -> Workspaces" works.
Closing old bugs