How can I get rid of every inappropriate stuff I saw on websites on my computer? And can I get rid of it or is it on there for good? ?

If the site in question has a 'report' function, you can use that to report any posts that violate the rules of that site. That doesn't mean the post will automatically be removed. Some sites have admins/moderators that review reports and decide if the post is a violation or not. YA requires several accounts report a violation before it's removed from the display. 

Nothing gets completely erased or deleted in cyberspace. I believe they save that stuff so that one day when things get slow they can go after you for something you wrote 20 years prior?

All that you can do is flag the comments that break the guidelines.  

This has NOTHING to do with Yahoo Answers.

You cannot change any content on ANY site you don't own. Deal with it.

Its not on your Computer its on the Website you should be Fine

Quora is a website. It is not on your computer. Unless you have malware there is nothing on your computer.

clear your browser history