Create a new blank branch (NEW_BRANCH)

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git checkout --orphan NEW_BRANCH
git rm -rf .

Add a single file (need to have a commit)

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touch README.md
git add README.md
git commit -m "initial"

Delete the branch you want to remove (OLD_BRANCH)

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git branch -D OLD_BRANCH

Rename the current branch (NEW_BRANCH) to the deleted branch (OLD_BRANCH)

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git branch -m OLD_BRANCH

Push your changes

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git push -f origin OLD_BRANCH