How to keep a window from overlapping into another workspace?

There doesn't seem to exist an option to do that in Compiz.

You can install the CompizConfig Settings Manager (package compizconfig-settings-manager, program ccsm) and raise the Edge Resistance Distance in the Snap Window plugin options in the Window Management section:

This will help to prevent pushing a window outside of the screen. However, if you move the window too fast it won't be triggered, so it's no optimal solution.

Maybe this issue is worth a mail to the Ayatana mailing list? :-)