First, to shut down Avast! Online Scanner, you can right-click the tray icon to close background services or killing its process by pressing Ctrl+Alt+Del to open task manager. But I think the safest place to force remove Avast! Online Scanner is in Safe Mode, which minimizes the chance to conduct wrong removals.
The second thing you need to do is to delete all Avast! Online Scanner files and clean up its registry keys. Normally going to Control Panel->Add/Remove programs can remove all major files as well as its relative registry, as long as you do it in Safe Mode. However, this can not give you a complete remove, you may still get Avast! Online Scanner services popping-up, and any attempts to re-install it or install other security software may be denied. If that is what happens in your case, you may as well try to find and locate and delete the files and directories.
Since the entries related to these files stay in separate parts of your registry and hard to pinpoint, you properly will risk a danger of messing up your registry while still ending up with a failing remove. I did not want this risk so I turned for a help when I wanted to force remove Avast! Online Scanner.
A small application named Best Uninstall Tool came as my final solution to force remove Avast! Online Scanner. It has a special method called Force Uninstall which can automatically look deep into your system and erase everything regarding the files and registry based on the directories and files you provide, meaning that you only need to locate the files mentioned above; the rest of the job will be quickly done by Best Uninstall Tool.