It varies from package to package. Many Steam games are installed with a normal Windows MSI that just targets your `$STEAM/steamapps/common` folder instead of `/Program Files (x86)`, and these packages can be removed using the Control Panel or CCleaner.
There are plenty of Steam packages that use the Steam infrastructure in a much more integral manner, however. These packages (like TF2 or Half-Life 2) are stored in compressed and encrypted cache files, and install only some components into a directory like `$STEAM/steamapps/$USERNAME/`. These packages can only be removed using the Steam application. While package stubs may be placed for the Control Panel to manage, the "uninstall" action will just call the Steam "Delete Local Content" action.
There are plenty of Steam packages that use the Steam infrastructure in a much more integral manner, however. These packages (like TF2 or Half-Life 2) are stored in compressed and encrypted cache files, and install only some components into a directory like `$STEAM/steamapps/$USERNAME/`. These packages can only be removed using the Steam application. While package stubs may be placed for the Control Panel to manage, the "uninstall" action will just call the Steam "Delete Local Content" action.