Fast User Switching provides a possibility to switch quickly beetwen users without loggin off. Follow these instructuins in order to disable or enable fast user switching option:
1. Go to "Start", "Run" type "services.msc" (without qoutes) and press enter.

2. Open "Fast User Switching" and then change "Startup Type" to "disabled" (this will disable loading service at next startup).

3. Right click on "Fast User Switching" and click on "Stop" (it will stop service immediately).
In case if you need to enable Fast User Switching just change "Startup Type" to "Automatic" and reboot your computer.