Right click on the header for the folder (e.g. Favorite Folder,
Folder). Be sure you right click on the header, not the actual folder.
Then, you will see a pop up menu. The third option is Display Tool Tips.
Use this to hide the info boxes. Each Folder group is independent, so
you have to do this for each one.
You can also change the icon sizes, add other information columns, and
change the sorting options.
To stop the info boxes over the actual emails, you go to Settings,
Configure Kontact, Mesage List, then check or uncheck “Display tooltips
for messages and group headers”.