I had a similar problem.
Here's how I fixed it: In the file ~/.kde/share/config/plasma-netbook-appletsrc, I searched for "opendesktop".
I got a hit like this:
I stopped KDE completely ("service lightdm stop" on a text console), removed those entries above (all sections starting with "[Containments][Applets]") and started lightdm again ("service lightdm start"). After that, I could switch to "Page One". It still had some empty applet on it. But I then were able to click on the Plasma icon in the lower left corner and completely remove the page.
Back on the "Start and launch" page, I simply added a new "Page One". The new page is empty and without those nasty social media BS choked down your throat. You can then configure it as you wish (I suggest to stay away from the OpenDesktop stuff though ).
There might be an easier way but now it's fixed for me.