How do I get rid of the MacKeeper virus?

It is not a virus. The main app has an uninstall option. That is the preferred way. Or read this:

Basics for Uninstalling Software

Most OS X applications are completely self-contained "packages" that can be uninstalled by simply dragging the application to the Trash. Applications may create preference files that are stored in the /Home/Library/Preferences/ folder. Although they do nothing once you delete the associated application, they do take up some disk space. If you want you can look for them at the above location and delete them, too.

Some applications may install an uninstaller program that can be used to remove the application. If you cannot find the uninstaller then you could download the freeware, Easy Find, to locate files that need to be removed. The places you need to search include:

1. /Home/Library/Applications Support/ folder. Check if the application has created a folder then delete the folder that's in the Applications Support folder.
2. Applications may install a startupitem or a LogIn item. StartupItems are usually installed in the /Library/StartupItems/ folder or the /Home/Library/StartupItems/ folder. Login Items are set in the Users & Groups preferences. Open it in System Preferences, then click on the Login Items tab. Select the item from the list and click on the Delete [-] button to remove it.
3. Some software use startup daemons or agents. Look for them in /Library/LaunchAgents/ and /Library/LaunchDaemons/ or in /Home/Library/LaunchAgents/.

Some applications install a receipt in the /Library/Receipts/ folder. Usually with the same name as the program or the developer. The item generally has a ".pkg" extension. Be sure you also delete this item as some programs use it to determine if it's already installed.

There are many utilities that can uninstall applications. Here is a selection:

1. AppZapper
2. AppDelete
3. Hazel
4. AppCleaner
5. CleanApp
6. iTrash
7. AppBolish
8. AppUninstaller
9. Uninstaller

For more information visit The XLab FAQs and read the FAQ on removing software.

for removing the ad quit Safari and relaunch it while holding down the SHIFT key. You may want to try Etrecheck or malwarebytesto search the system for adware or malware.

For futher analysis you may post an Etrecheck report as well

and going forward avoid sites that advertise MacKeeper.

for removing the application see this article

https://www.macworld.com/article/2861435/software-utilities/how-to-uninstall-mac keeper-from-your-mac.html

Mackeeper is not a virus, it's just garbage software

MacKeeper is not a virus, it's just a regular piece of junk that the user had to install. It has an uninstaller, you should use that.