Steps to clear out the history of a git/github repository

git-clearHistory

-- Remove the history from 
rm -rf .git

-- recreate the repos from the current content only
git init
git add .
git commit -m "Initial commit"

-- push to the github remote repos ensuring you overwrite history
git remote add origin git@github.com:<YOUR ACCOUNT>/<YOUR REPOS>.git
git push -u --force origin master

Is it possible to delete the history from within Github? Thanks.

Yes. You need to contact GitHub support and they can run gc on the GitHub repo. Everything that is not reachable from a branch or tag will get cleaned up.

The instructions above are all around creating new branches that don't connect to the old history. So the old history will be eligible for gc (garbage collection). Keep in mind that aside from branches and tags, PRs and Issues might have links to old history and so they will prevent gc.