For packages downloaded from the official site: Find the unins000.exe file in the installation path and run it or use the Control Panel on your computer to uninstall.
For packages updated by the client: Use the Control Panel on your computer to uninstall.
If files are in use, they will not be deleted and you will have to delete them manually.
Solution:
Open the Task Manager to see if there is a process using any ArtLib files. If there is, end the process, then uninstall ArtLib.
If the previous solution doesn't workout, try restarting your computer and uninstall again.