The MacKeeper installation file is often found in most Mac users' Downloads folder, often multiple copies of it, as a consequence of loading a web page (or being redirected to one) that initiates the download immediately.
Drag the downloaded MacKeeper .dmg file or installer package to the Trash. As long as you did not install it – which requires your consent – it can have no effect.
The Guest User account is a consequence of using Find My Mac. If you don't want to use it, disable it in System Preferences > iCloud.
Or, you configured a Guest User according to OS X Yosemite: Set up users on your Mac. If you don't want it, disable it, then log out or restart your Mac.