A re-install is way OTT for this! Indeed, I'm surprised that it bugs you so much.

I've not come across it, but if you look in Kmenu>Settings>Control Center>Session Manager you'll see a variety of options under On Login: experiment with those. You might even try deleting both of the trash cans (but make sure you know how to re-create one first!

I think it's a Kde bug: if you do a google there might just be an answer on the Kde site.