It's a kludge, but I fixed it like this:
-rename /home/itl/.kde4/share/apps/activitymanager and /home/itl/.kde4/share/config/activitymanagerrc
-log out and back in
-kill apps which are running, but not visible from the command line
-set wallpapers back to the way they were before all this started
-save the session, so invisible apps aren't started in the future
About the last item- I always check "restore manually saved session" in "system settings-> startup and shutdown-> session management". This was causing hidden apps to be started every time I logged in.
I have to say- although the concept of activities makes sense in theory, The implementation is difficult for me to understand. Maybee the pager could be modified to make getting to wherever the hidden apps are more intuitive. At the very least, there should be some button that restores things to the way they were.