You can't "block" a website because the only way to see a website should be to visit it yourself. So in this situation, it appears that something else is going on.
A search for "onlineMapFinder" turns up some information that suggests it is a toolbar or adware that you must have installed on your Mac. Search for "Mac Remove OnlineMapFinder" and you come up with many suggestions about how to remove/uninstall it.
It sounds like it may just be a browser extension or toolbar that you can easily remove. You don't mention which browser you are using, so look in the preferences/settings for that browser for extensions and toolbars and see if it is there.
You can't "block" a website because the only way to see a website should be to visit it yourself. So in this situation, it appears that something else is going on.
A search for "onlineMapFinder" turns up some information that suggests it is a toolbar or adware that you must have installed on your Mac. Search for "Mac Remove OnlineMapFinder" and you come up with many suggestions about how to remove/uninstall it.
It sounds like it may just be a browser extension or toolbar that you can easily remove. You don't mention which browser you are using, so look in the preferences/settings for that browser for extensions and toolbars and see if it is there.